Bug-2590: Clustering : Minimize usage of in-memory journal

In order to minimize the memory usage of the in-memory journal, we can remove the entries from the Leader's journal
once it has been successfully replicated to ALL its followers.
This does not intefere with snapshots, as we capture snapshots on demand.

The followers follow the leader in cleaning the in-memory journal, there by ensuring that all the journals have more or less same entries.
This is done by the leader passing its replicatedToAllIndex as part of the AppendEntries.

Change-Id: I579a1f90d3c4e5d6be4ce699072688788b07bd48
Signed-off-by: Kamal Rameshan <kramesha@cisco.com>