Bug 7521: Convert Snapshot to store a State instance

Bug 7521: Convert Snapshot to store a State instance

Created a new Snapshot class that stores the state as a Serializable
State instance instead of a byte[] to lay the groundwork for handling
snapshots > 2G. In this manner, State implementations can serialize
their data directly to the ObjectOutputStream.

The previous Snapshot class was deprecated but, unfortunately, it
can't simply "read resolve" to the new class on de-serialization
as the deserialization code doesn't know how to convert the data
from byte[] to a State instance. Therefore a new method,
deserializePreCarbonSnapshot, was added to RaftActorRecoveryCohort.

The introduction of the State interface necessitated changes throughout
the snapshot capture and snapshot recovery code paths. The Shard
implementations were also changed accordingly.

For follower snapshot install, an optional OutputStream is also passed
to createSnapshot. The new API contract is: if the OutputStream is present,
the implementation must serialize its state to the OutputStream and return
the stream instance in the CaptureSnapshotReply along with the snapshot
State instance. The snapshot State is serialized directly to the snapshot
store while the OutputStream is used to send the state data to follower(s)
in chunks. The deserializeSnapshot method is used to convert the serialized
data back to a State instance on the follower end. The serialization for
snapshot install is passed off so the cost of serialization is not charged
to the raft actor's thread.

The OutputStream is converted to a ByteSource when passed to AbstractLeader.
AbstractLeader still converts it to a byte[] but will be changed to use
the ByteSource's InputStream to chunk the data in a subsequent patch.

Also the SnapshotManager currently passes a ByteArrayOutputStream to
createSnapshot but this will be changed to a FileBackedOutputStream in a
subsequent patch.

Change-Id: I2ea30870b54478d7ef5669335ca4b444539f8d56
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>