Solution: cast the unused variable

* Problem: radix tree needs benchmarks and improvements

Solution: add a benchmark and make suggested improvements

subscriptions increases

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

- Lowercase all commands
- Unify indent to 2 spaces
- Remove spaces around brackets
- Remove repitition of condition in else(...) and endif(...)

Note: (re-)running CMake did not change the content of the generated files

Use ZeroMQ_SOURCE_DIR, ZeroMQ_BINARY_DIR instead of CMAKE_SOURCE_DIR, CMAKE_BINARY_DIR, which allows libzmq to become an "add_subdirectory" target (with static build).
Otherwise, building unittest_xxx would complain header files not found.

Solution: added test case, reproduces the likely cause for #3107

Solution: make proper boolean expression

Solution: Add IPv6 support

for multicast

Solution: augment the UDP URL syntax to accept an interface specifier with a
syntax similar to the PGM urls.

Fixes #2212

Solution: factor the code into ip_resolver, add IPv6 support and unit tests.

Solution: replace it with the ip_resolver code shared with the TCP
address code

It simplifies the UDP parsing code and makes it behave more like the
TCP counterpart, in particular it's not possible to connect to hosts
by name and bind by NIC names.

It also adds support for "*" port resolving to 0 (useful to let the OS
allocate the port number).

Solution: add unit tests

Solution: return an error in this situation but still allow using an explicit
"0" if somebody really wants to connect to port 0.

This shouldn't break any existing code because a "*" port was already rejected
in an early test in the TCP path in zmq::socket_base_t::connect.

Solution: Factor the code into a different file with a well defined API and add
unit tests.

Solution: removed

Solution: correctly dereference pointer in test.

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

Solution: added test case

Solution: introduce a type template argument

Solution: added further test cases

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 some more test cases

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

Solution: fix order of statements in test_events_t

reuse

Solution: extract into functions defined in ip.hpp

Problem: signaler_t::make_fdpair not reusable

Solution: move make_fdpair to ip.hpp

Problem: epoll worker with no fds cannot be stopped

Solution: use interruptible epoll_pwait call

Problem: insufficient unit tests for poller

Solution: add test cases

Solution: set up initial unit tests