conncheck: wait for a pair until all stun requests are completed

Only the newest stun request may need to be retransmitted, according to the pair retransmit flag. This is the first element of the stun_transactions list. Older stun requests are just kept around until their timeout expires, without retransmission.

The newest stun request is usually the last one that will timeout. Current code was based on that assumption, causing the pair to fail when the newest stun request timeout expires. This is not always true, and some older stun requests may have a greater timeout delay.

So, we should wait until all stun requests of a given pair have reached their timeout.

We also refactor this part of the code, to handle the first stun and the other stun requests in the same loop.