CDS Frontend client actor should delete prior snapshots

The RecoveringClientActorBehavior increments the last generation id and
saves a new snapshot. However the prior snapshots remain in akka
persistence - every time the controller is restarted a new snapshot file
is created. We should delete the prior snaphsots.

The snapshot file names were very long with escape chars b/c
FrontendIdentifier.toString is used for the frontend client actor's
persistence ID. We should use a shorter, more readable ID, so I changed
it to the form:

  member-1-frontend-datastore-config
  member-1-frontend-datastore-operational

Change-Id: I1c77c826729ca1a36497a1236ac99f7cc77efb72
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>