QBFT fast recovery by frequent multicasting of RC message

[pullurib]

PR description

• When a node moves up a round, e.g. from 6 -> 7, it only sends a single round-change message
• This means that if a node misses the round-change message from another node it doesn't know the node is on round 7
• Nodes can publish their current round every N seconds while they are still in that round

Enabled by the experimental option --Xqbft-fast-recovery . It's very effective when combined with --Xqbft-enable-early-round-change

[pullurib]

• Added a Frequent multicaster class to resend a message to validators at regular intervals . This is used for round change messages upon using fast-recovery option
• Moved early-round-change , fast-recovery and frequent multicaster to final state
• Testcases for frequent mutlicaster unit tests and ATs for new option.

[macfarla]

could this be a plugin?

[pullurib]

@matthew1001 is working on moving QBFT functionality to a plugin . This would be a part of that plugin then.