Fix a bug in the client.

If a new config was published, it would get stuck in a loop forever
being notified of the new config and never actually fetching it.
Re-deliver messages that arrive too early.
Add sequence numbers to logged transfer state
Set has_value to true in decode_key_value
Check for retransmissions first to log less false positives.
Fix the replication stress test and make it pass
Make (req_id, seq_id) -> (ri, req_id, seq_id)
Block signals in background threads
Garbage collect (req_id, seq_id) pairs