NAKACK2: retransmit the last-message-missing sooner

When we multicast messages (with NAKACK2), if the last message M is dropped (e.g. because the receiver has a full thread pool), then it will take a while for M to be retransmitted.
Currently, detection of missing messages is done in STABLE: this protocol periodically (or based on receiving a certain number of bytes) broadcasts its digest. When the coordinator has received all digests, it computes the min and broadcasts a STABILITY message.
When members receive this message, they can purge all messages below the minimum vector. Receivers also detect if the digest contains messages higher than the ones they actually received and can thus perform retransmission, if needed.
So the main purpose of STABLE is purging of messages seen by everyone, not detecting missing messages. It also takes a while (consensus from all members) to generate a STABILITY message, so detecting a missing message takes long.
We therefore need a quicker way to detect and retransmit missing messages.

Solution

• Add a flag to NAKACK2 (not STABLE) which - if set - periodically (or based on the number of bytes sent), broadcasts the seqno of the highest message it sent.
  • xmit_interval can be reused as interval
• When a receiver detects that it didn't receive this message, it can ask the sender for retransmission
• The task should acquiesce when no messages are missing (i.e. the highest delivered message == the highest received message)
  • When the current seqno hasn't changed for N times, don't send the highest seqno
• When messages are sent, the task should not run either
• It should also be possible to trigger the sending programmatically, or via JMX