Solution: Explicitly explain message ownership semantics when the call
fails.

Fixes #3526.

Add missing information on multicasting with high data rates to PGM docs

Solution: Include a new section on configuring both ZMQ and the host
OS tx/rx buffers to facilitate sending large messages at a high data
rate with the PGM protocol.

Solution: Added relicense agreement

acinclude.m4, configure.ac: Fix several non-portable uses of `test`

Fixes #3517

Fix Fedora 30 build

Implement thread_t::setThreadName() on windows

Problem: clang-format 5 has issues with comments

Problems: typos in docs, unnecessary ifdefs, duplicated APIs

Solution: now that clang-format 6+ is used in the CI, fix it

Solution: bump CI to version 7

Solution: unify the two APIs, as they are both still in DRAFT state and
thus can be changed.

Solution: fix them

Solution: use unlikely to optimize for the most common case

Solution: remove them, only public headers have to be ifdef'd

Solution: list it

 Support XPub socket send last value caching to last subscription pipe with ZMQ_XPUB_MANUAL_LAST_VALUE. (#3511)

* Add ZMQ_XPUB_MANUAL_LAST_VALUE

* Surpport xpub send last value caching to one pipe with ZMQ_XPUB_MANUAL_LAST_VALUE

* Add test_xpubub_manual_last_value

* Add relicense and doc

Problem: Potentially unitialized variable in #3508

Solution: Create a unique endpoint for each fcn call.

Solution: Initialized it

Problem: Socket monitoring only allows ZMQ_PAIR

Solution: Allow ZMQ_PUB and ZMQ_PUSH sockets types for the monitoring.
This way someone could create a ZMQ_PULL socket connected to multiple
monitoring sockets at the same time.

Problem: PLAIN HELLO message incorrectly uses WELCOME literal

Solution: change to HELLO

Problem: signature of zmq_poller_fd does is incompatible with regular…

Solution: change return type to int (again) and return fd via an output parameter

Problem: syntax error on Windows related to socket descriptor type

Solution: use proper fd_t type, and also define and use a zmq_fd_t in the API

Problem: generated ZeroMQConfig.cmake does not work when only static …

Solution: check for availability of shared/static libzmq targets

Some cleanup in CMakeLists.txt

Solution: use check_cxx_symbol_exists resp. check_type_size instead

Solution: Group include directives together at the beginning of the CMakeLists.txt file

Solution: remove it

Solution: fix spelling

Solution: remove redundant debug output