Allow AbstractClientActor generation to start from non-zero

Allow AbstractClientActor generation to start from non-zero

This adds the ability to override the initial generation seeded
into persistence. The requested generation is controlled via
system properties on a per-client-type basis:

org.opendaylight.controller.cluster.access.client.initial.generation.datastore-operational = 500

This is useful during bringup of a zombie node, which lost all
of its persistence, for example during recovery from backup.

JIRA: CONTROLLER-1626
Change-Id: If32487fedcb5b6b39551a3359c069df940a6a725
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>