conn_check_prune_socket() was trying to access deleted NiceCandidate
instance. Prune the socket before removing the candidate.

Closes #73

Call nice_agent_peer_candidate_gathering_done () where appropriate.

The STUN thread may be blocked waiting for stun_signal_ptr so signal it
when stopping the thread.

In this mode, activated by setting "ice-trickle" property of a
NiceAgent to TRUE, a component without a successful connectivity check
will wait indefinitely for more remote candidates to come.

nice_agent_peer_candidate_gathering_done() must be called after
receiving the last remote candidate for the component to finally change
its state to NICE_COMPONENT_STATE_FAILED.

Windows users should just use Meson for building libnice.

Forgotten to add in 0.1.15 release?

While waiting for write thread to finish sending, keep also receiving so
that any STUN messages from the peer still get processed.

Closes #71

Limits accidental leakage of secret information.

Don't set a timeout to the very far future if the lifetime sent  by the
server is under 60s. Instead of doing "lifetime - 60", switch to lifetime/2.

Don't unref NiceAgent while holding agent mutex. If it's the last
reference, nice_agent_dispose() will try to acquire the mutex again and
deadlock.

Also remove workarounds for the older versions.

coturn doesn't authenticate us before sending us to an alternate server.

glib documentation says G_SOURCE_FUNC() macro should be used to suppress
the warning about incompatible function types when the function will be
cast back to the correct type before it is called by the GSource.

An application using more than one NiceAgent instance may crash due to
a race in gUPnP.

Fix is to be released in gUPnP 1.1.2 / 1.0.4:

https://gitlab.gnome.org/GNOME/gupnp/commit/0123e574595e0a547ce26422633df72d63d3d0e0

libnice.so created by autotools has its symbols bound to the default
'Base' version node, whereas Meson build arbitrarily introduced a
version node named 'libnice'. This breaks tools like dpkg-gensymbols
that track changes in exported symbols through several releases of the
library.

Remove 'libnice' version and revert back to using unspecified version.

Errors reported by lintian -EvIL -pedantic.

This is because Docker doesn't have IPv6 by default.