BUG-5280: persist metadata in snaphots

BUG-5280: persist metadata in snaphots

Note: this patch needed to be cherry-picked before
https://git.opendaylight.org/gerrit/#/c/45028/ coild be cherry-picked.

This patch adds the wiring in ShardDataTree to persist
various pieces of metadata in a snapshot. It also includes
metadata recovery from a snapshot.

In order to make this work, this patch centralizes all
actual payload and snapshot handling within the ShardDataTree
by introducing explicit entrypoints for each avenue through
which data can be introduced.

Change-Id: Ibc15bd152bd44dd583d67bb7fc61bc8f3086df30
Signed-off-by: Robert Varga <rovarga@cisco.com>