Send commitIndex updates to followers as soon as possible

Send commitIndex updates to followers as soon as possible

When the commitIndex gets updated when we reach consensus, we should
propagate this fact to the sender peer. Otherwise data tree change
listeners on peers may be unnecessarily delayed, as they end up
waiting for heartbeat interval (or non-batched entries) to be propagated
out.

The results in observably-better behaviour between leader and follower,
as the leader is sending out notifications of consensus, without
delayling for heartbeat -- which is reflected in the changes to
LeaderTest.

Furthermore, DelayedMessagesElectionScenarioTest is detecting the change,
as two members may achieve asynchronous consensus -- hence the leader
would be sending two messages (request to persist, commitIndex) to push
the consensus out. This exposes a race in the test, where we do not know
the order between TimeoutNow and AppendEntries. If AppendEntries is
processed by member 2 after TimeoutNow, leader will know about the new
term due to AppendEntriesReply we send out. The cluster will still
converge, but the assert for leader will be different if it manages to
process the message. Fix this by isolating member 2.

JIRA: CONTROLLER-1900
Change-Id: I695ef25c7a4cf8799c9c5e04c2c33fbf3e2f21df
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>