Refactor persister: Add ability to publish multiple snapshots while loading initial...

Refactor persister: Add ability to publish multiple snapshots while loading initial configuration.

Persister adaptors now return list of snapshots to be pushed to netconf at server startup. Persister maintains an ordered list of adaptor
instances, each with its configuration. During server startup it iterates the list backwards until non empty list of snapshot is returned.
Each snapshot can depend on different capabilities, once all capabilities for given snapshot are announced by server, persister pushes this
snapshot.

Change-Id: If73ead980c9cf8cd237af170872fbf1a491cb029
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>