conncheck: set writable callback to socket from TCP active connect

conncheck: set writable callback to socket from TCP active connect

A new socket created in nice_tcp_active_socket_connect() should have its
writable callback set, because it's possible for it to become a base
socket of a peer reflexive candidate, if some is discovered by
connection checks on that TCP active candidate.

Previously, when such prflx candidate became selected, without write_cb
on the socket the agent was never notified about it becoming writable
again after the socket's buffer got filled up. This caused the data flow
to hang permanently.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Olivier Crête <olivier.crete@collabora.com>
http://phabricator.freedesktop.org/T117