raft: bound leadership transfer time with blocking/throttling

[dotnwat]

If a leader is handling raft operations leadership transfer may be delayed. If such operations are capable of starving the leadership transfer process, then a throttling mechanism is required to support forward progress of leadership transfer.

In principle this may not be needed for acks=all because followers will be up-to-date and leadership transfer will be just another raft operation that will complete. However, for acks=0,1 the leader may be far ahead of the followers. Since followers need to be caught up before leadership transfer completes, on-going writes may result in starvation of the transfer process.

• Apply back pressure for throttling
• Block operations fully if within some bound

Additional notes

• RFC https://docs.google.com/document/d/1FcHekvM62RYRZqZaIvHbKucIQSGuSU2uuP6RW5r-oko/edit#
• PRD https://docs.google.com/document/d/140T_4Rg9cfIaTuPNlsEW-nfZ3INiLBHUxFy4-HsB3CY/edit#

JIRA Link: CORE-787