Solution: extract functionality into send_routing_id

Solution: extract functionality into get_effective_conflate_option

Solution: use a std::vector instead

Solution: introduced constant

Problem: inconsistent naming style for private data members, conflicts with naming of local variables and member functions

Solution: apply and check _lower_case naming style for private data members

Solution: configured clang-tidy check and applied fixes

Solution: define and apply parameter naming style: lower_case_

Solution: replace by C++-style casts

Solution: do it explicitly to fix build on Solaris 10/11 with the Sun
compiler

The zero copy decoding strategy implemented for 4.2.0 can lead to a large
increase of main memory usage in some cases (I have seen one program go up to
40G from 10G after upgrading from 4.1.4). This commit adds a new option to
contexts, called ZMQ_ZERO_COPY_RECV, which allows one to switch to the old
decoding strategy.

Solution: extract thread_ctx_t from ctx_t

Solution: applied clang-format

Solution: signal error to caller, and apply appropriate cleanup

Change ZMQ_THREAD_AFFINITY to ZMQ_THREAD_AFFINITY_CPU_ADD/ZMQ_THREAD_AFFINITY_CPU_REMOVE. Avoid prefix thread names when no prefix was set.

Solution: use std::map::emplace instead, where available

* Add ZMQ_THREAD_NAME_PREFIX ctx option

* Background thread scheduling 

- add ZMQ_THREAD_AFFINITY ctx option; set all thread scheduling options
from the context of the secondary thread instead of using the main
process thread context!
- change ZMQ_THREAD_PRIORITY to support setting NICE of the background
thread when using SCHED_OTHER

Solution: replace by "routing id"

Solution: replace by "routing id"

Solution: add a crypto [de-]initialiser, refcounted and serialised
through critical sections.
This is necessary as utility APIs such as zmq_curve_keypair also
call into the sodium/tweetnacl libraries and need the initialisation
outside of the zmq context.
Also the libsodium documentation explicitly says that sodium_init
must not be called concurrently from multiple threads, which could
have happened until now. Also the randombytes_close function does
not appear to be thread safe either.
This change guarantees that the library is initialised only once at
any given time across the whole program.
Fixes #2632

Solution: don't set thread name on Android

Setting a thread name on Android may fail with "permission
denied" error and abort the process due to failed assertion.
Tested on Android 5 and 6 (two phones).
Strangely enough, it only happens on signed APKs and is fine
in debug. Using JeroMQ is not an option as we need TCP keepalive
settings and authentication which JeroMQ doesn't support.

Solution: call it simply "ZMQ background" instead of "ZMQ b/g thread"

Solution: use pthread API to set the name. For now call every thread
"ZMQ b/g thread". Would be nice to number the I/O threads and name
explicitly the reaper thread, but in reality a bit of internal API
churn would be necessary, so perhaps it's not worth it.
This is useful when debugging a process with many threads.

Solution: add a ZMQ_MSG_T_SIZE context read-only option so that
wrappers can call zmq_ctx_get (ctx, ZMQ_MSG_T_SIZE) to get the
size at runtime.

Solution: add a zmq_assert to check if the ephemeral sockets created
to drain the queue of pending inproc connecting sockets was allocated
successfully.

Problem: zmq_ctx_term asserts with connect-before-bind and sockets with identity over inproc transport

Solution: check if the connecting inproc socket has been closed
before trying to send the identity.
Otherwise the pipe will be in waiting_for_delimiter state causing
writes to fail and the connect to assert when the context is being
torn down and the pending inproc connects are resolved.
Add test case that covers this behaviour.

Solution: replace tabes with spaces

Convert manual (locking and unlocking) mutexes to scoped mutexes for the case of unlocking mutex even if the protected operation throws an exception (#2071)

Since ZMQ_USE_SODIUM is nowhere defined i guess it's meant to be ZMQ_USE_LIBSODIUM?

I changed this to ZMQ_USE_SODIUM to be consistent with other
configuration options (especially ZMQ_USE_TWEETNACL).

Solution: fix it.

Solution: use ZMQ_USE_LIBSODIUM to match ZMQ_USE_TWEETNACL

Solution: use proper preprocessor macro

libsodium calls abort() when /dev/urandom can't be found
even if one creates ZeroMQ context before calling chroot()[1].

This happens because crypto gets initialized on handshake,
and at that moment the process is already chroot'ed.

Solution: initialize cryptographic libraries in ctx

randombytes_close() is already there in the destructor.

[1] https://download.libsodium.org/doc/usage/index.html

Solution: Phase I - make precompiled.hpp be first file included in every source file

- they have no copyright / license statement
- they are in some randomish directory structure
- they are a mix of postable and non-portable files
- they do not conform to conditional compile environment

Overall, it makes it rather more work than needed, in build scripts.

Solution: clean up tweetnacl sauce.

- merged code into single tweetnacl.c and .h
- standard copyright header, DJB to AUTHORS
- moved into src/ along with all other source files
- all system and conditional compilation hidden in these files
- thus, they can be compiled and packaged in all cases
- ZMQ_USE_TWEETNACL is set when we're using built-in tweetnacl
- HAVE_LIBSODIUM is set when we're using external libsodium