Problem: enormous memory increase due to zero copy decoding

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.

* fix pthread condvar timeouts

This fixes a race condition / hang for threadsafe sockets.
Context: https://github.com/zeromq/czmq/issues/1873#issuecomment-370279244

Problem: unittest_mtrie fails on 32bit *nix

Solution: correctly dereference pointer in test.

Problem: no unit tests for mtrie

Solution: Return an enum from rm instead of a bool, and adapt existing uses

Solution: generalized terms to entries/values

Solution: added test case

Solultion: clarify that notifications for unique subs/unsubs are always
passed, and these option only affect the behaviour for duplicated ones.

Solution: add test_xpub_verbose to cover those APIs

Solution: introduce a type template argument

Solution: added further test cases

Solution: added some TODO comments, improved existing documentation

Solution: added test case, reduced code duplication in tests

Solution: added unit test and assertions

Solution: add test case

Solution: added some test cases for rm

Solution: added test case

Solution: add CODE_COVERAGE_*FLAGS to unit tests build

Fixes #2949

Solution: add some more test cases

Solution: made mtrie generic (to remove complex dependency on pipe_t),
and added some unit tests

Solution: add const, introduce typedef

Fix gcc 8 build warning/error with -O3.

Problem: No support for addressing using TIPC Port Identity

ZeroMQ currently supports location independent addressing using TIPC
Port Names with tipc://{type,instance}. This commits adds support for
connecting and binding using TIPC Port Identity addresses. To connect
using Port Identities the expected format is tipc://<Z.C.N:Ref>, e.g.
"tipc://<1.2.3:123123>". To bind using TIPC Port Identities the expected
format is "tipc://<*>".

Problem: race condition in unittest_poller

Minor fixes for windows cmake build

This fixes an error with the cmake install configuration, which
attempted an invalid copy of a .pdb file on windows, when the
BUILD_SHARED option is disabled.

This adds a new cmake build option called BUILD_TESTS, that can be used
to enable/disable building of the tests. This is enabled by default.

This updates the cmake set_target_properperties usage, so that the suffix
is specified for the MinSizeRel build type.

Solution: fix order of statements in test_events_t

Fix build break when SIO_LOOPBACK_FAST_PATH is not defined (i.e. in VS2010)

Problem: insufficient unit tests for poller concept and ypipe

Solution: apply clang-format

Solution: fix shutdown of kqueue_t

Solution: use std::atomic only from VS 2015

Solution: use shallow_clone option instead of clone_depth