Bug 7521: Add custom local snapshot store

Bug 7521: Add custom local snapshot store

Akka's LocalSnapshotStore serializes to a byte[] before persisting
to the file so we need to write our LocalSnapshotStore that serializes
directly to the file. I patterned the code after akka's LocalSnapshotStore,
mainly translated from scala.

Akka's LocalSnapshotStore wraps the payload data in a Snapshot class
and uses the SnapshotSerializer which locates the serializer for the payload
class and writes some header data before delegating to the payload
serializer. To handle backwards compatibility for a snapshot previously
serialized with akka's format, java de-serialization will detect and fail
with an invalid stream header, in which case we fall back and try
de-serialization via the SnapshotSerializer.

Akka has a standard test suite in SnapshotStoreSpec for testing custom
snapshot store plugins. I derived a LocalSnapshotStoreSpecTest class
that does the setup and teardown with SnapshotStoreSpec doing the rest.
SnapshotStoreSpec uses ScalaTest stuff so need to be run with scala's
JUnitRunner.

I also added a regular LocalSnapshotStoreTest class to cover a few cases
that SnapshotStoreSpec doesn't.

Change-Id: I1ca11682f37aa39d60d3ce57c874c299627e8ca6
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>