Fix bug in ReplicatedLogImpl#removeFrom and use akka-persistence for removing entries

The way things were coded in removeFrom assumed that when a set of entries was deleted
from the persistent log the sequence number would be reset. This is most likely to not be
the case.

Switched to usage which is more appropriate for akka-persistence namely to persist
the operation to delete entries in the akka-persistence journal and on recovery take the
appropriate action.

Also since now we are using the akka-persistence journal purely for persistence we can safely
add other stuff to the journal and so I added UpdateElectionTerm also to the journal. This
takes care of persisting the election information in the same journal.

Change-Id: I34202535711660f19b4cdd9e71a7b55fde7433ce
Signed-off-by: Moiz Raja <moraja@cisco.com>