BUG-5280: centralize ShardSnapshot operations

BUG-5280: centralize ShardSnapshot operations

Current shard snapshotting mechanism does not allow for evolution
of the snapshot contents and contains only the root node. Also
the serialization and deserialization operations are scattered
in multiple places, making coordinated changes a bit troublesome.

This patch introduces a versioned snapshot abstraction and moves
serdes operations into a single place. A new serialization format
is introduced, which holds the root node and some additional
metadata. No concrete metadata is defined in this patch, but this
will be used to transfer frontend protocol state from shard leader
to shard follower.

It also moves the act of creating a snapshot into ShardDataTree
and creates a dedicated actor to handle the snapshotting task,
which is used for all snapshot requests for a particular Shard.
Also makes the actor message internal to the ShardSnapshotActor,
providing a convenience method to create and dispatch it.

Change-Id: I6d9680b6ef08672c363092a649255013980c0bd6
Signed-off-by: Robert Varga <rovarga@cisco.com>