BUG-8618: introduce RaftActor.unpauseLeader()
This is a preparatory patch, which notifies RaftActor when
the operation hooked to pauseLeader() fails to complete and the
leader should resume its normal operation.
This is needed to correctly resume operations of tell-based protocol
after a pauseLeader() completes without actually changing the leader.
Change-Id: Ia00e52ebb327575a484af62bf0c31131a33303b3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
3a10a45e0f78337435c8bc84015c4724a9fa7741)