conncheck: Only valid stun messages used for Keepalive

conncheck: Only valid stun messages used for Keepalive

Previously, a STUN response for which there was no associated request
would be considered valid media input and as such could keep a dead
connection alive. If peer A was communicating with peer B and peer B got
disconnected, the keepalive mechanism in peer A should detect this.
However, a misbehaving STUN server could keep sending STUN responses to
peer A which would then be considered a valid communication between A
and B and thereby prevent the keepalive mechanism from shutting down the
connection.

Situation above refers to a stun message validated as
STUN_VALIDATION_UNMATCHED_RESPONSE. With this change only messages
validated as STUN_VALIDATION_SUCCESS may keep the connection alive.