Bug-4214 - Add support for configurable snapshot chunk size.

Bug-4214 - Add support for configurable snapshot chunk size.

Added a new variable in the distributed-datastore-provider.yang. This
will be used to configure the snapshot chunk size. Added various
setters/getters to the DatastoreContext. The support for this variable
was added to JMX as well, so that the value can be seen via JConsole.
Moreover, added tests in DatastoreContextTest.

Also fixed a recurring typo in sal-akka-raft. Snapshot was spelled as
snaphot (missing s in shot).

This code was unit tested with different entries in datastore.cfg. Also
tested the case where no special value was provided in datastore.cfg,
and the default value was shown in jconsole.

Change-Id: Ie754075cc25f9eadf01cc65aee726735144c1794
Signed-off-by: Shaleen Saxena <ssaxena@brocade.com>
(cherry picked from commit f3c38988cc31e07bab473cace946aebf5152c61f)