* Problem: radix tree needs benchmarks and improvements

Solution: add a benchmark and make suggested improvements

Problem: #3263 not mentioned in NEWS

Solution: add it

Problem: using FD_SETSIZE for optimized_fd_set_t is a pessimization

Reapply session_base_t refactoring

Solution: do not use make_pair

Windows apparently returns `WSAENOBUFS` from `recv`

Problem: Assert being triggered when Windows returns `WSAENOBUFS` from `recv` call on large buffers.

Solution: Expect `WSAENOBUFS` as a valid response.  See https://github.com/zeromq/libzmq/issues/3263

Solution: added relicense agreement

Solution: extract functions for each protocol

Problem: build broken with Sun Studio, tests fail on debian/kfreebsd and debian/hurd, DRAFT zmq_poll is slow

Solution: remove it

Solution: use it only if there is at least one thread-safe socket,
which is not compatible with the older zmq_poll implementation.

Solution: define UNITY_WEAK_PRAGMA when building with the Sun compiler
as it does not support the weak attribute.

Solution: skip tests that use ipc://* as it's impossible to get the
address back for the client

Solution: temporary revert refactoring until it can be properly fixed.

Revert "Problem: complexity of start_connecting"

This reverts commit 6d7aeb05.

Solution: use a different system variable for debian/hurd and
debian/kfreebsd so that the tests are skipped only on hurd.

Solution: fix it

Problem: stale bot didn't parse labels correctly

Solution: quote the labels with a space, upstream documentation
(not displayed on the Github page) suggests this should finally work

Problem: stale issues linger on Github

Solution: enable Stable Bot to automatically mark an issue as stale
after 365 days of inactivity and close it after further 56 days.

Issues marked with the following labels are excluded:

 - Help Request
 - Feature Request
 - Problem reproduced
 - Critical

Problem: NEWS still out of date

Solution: mention all relevant bug fixes, features and changes

Problems: wrong version and outdated NEWS

Some changes for CMakeLists.txt:

- MacOS version requires CMake version 3.0.2 (because of policy CMP0042)
- Add option to build instrumented binaries with Address Sanitizer
- Add option to select compiler intrinsics for atomic ops
- Only build docs on request (saves build time)

Solution: partially list changes since 4.2.5. Not yet complete.

Solution: bump back from 4.3.1 to 4.3.0

Problem: new radix tree implementation is experimental

Solution: use it only for DRAFT builds for now, to leave it time to be
tested and stabilised.

Problem: potentially large memory footprint of trie as number of subscriptions increases (issue #1400)

Solution: add it

subscriptions increases

Solution: use a radix tree instead of a trie to store subscriptions

Problem: Use of pipe_t after free in zmq::socket_base_t::term_endpoint(). Issue #3245.

Solution: When pipe_t is freed (terminated) remove it from _endpoints member of zmq::socket_base_t. Resolves issue #3245.

Add support for UDP multicast ttl (ZMQ_MULTICAST_HOPS)