Lost commit index when a snapshot is captured

Lost commit index when a snapshot is captured

When we have no journals and a snapshot is recovered and
immediately captured again, if there was no movement in the
datastore we loose the lastIndex and term from the previous snapshot.
We can safely reuse these from the previous snapshot when no
entry is present in the ReplicatedLog.

Change-Id: Iaeb71edc7ec865bec18dbcb436af76e592eea69d
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit aad11d48fcace37e3365388f387e16fa67257a25)