Introduce a new verb dedicated for receiving and sending hints: HINT_MUTATION. It is handled on the streaming connection, which is separate from the one used for handling mutations sent by coordinator during a write.
The intent of using a separate connection is to increase fairness while handling hints and user requests - this way, a situation can be avoided in which one type of requests saturate the connection, negatively impacting the other one.
Information about new RPC support is propagated through new gossip feature HINTED_HANDOFF_SEPARATE_CONNECTION.
Fixes#4974.
Tests: unit(release)
3a46b1bb2b