Pairs with the state NICE_CHECK_CANCELLED are the pairs targeted for
removal after the nomination of a pair with an higher priority,
described in Section 8.1.2 "Updating States", item 2 of RFC 5245. They
include also pairs that overflow the conncheck list size, but this is a
somewhat more marginal situation. So we are mainly interested in the
first use case of this state.

This state mixes two different situations, that deserves a distinct
handling : on one side, there are waiting or frozen pairs that must be
removed, this is an immediate action that doesn't need a dedicated state
for that. And on the other side, there are in-progress pairs that
should no longer be retransmitted.

This patch removes the cancelled state, and adds a flag
stop_retransmit_on_timeout to deal with this last situation.

Differential Revision: https://phabricator.freedesktop.org/D1114