Bug -3221 : Adding a new DataStoreUnavailableException for external applications.

If cluster member, which is a leader of one or multiple shards becomes unavailable,
Shard re-election happens after certain time period and anyone tries to access datastore,
receives akka.timeout exception. No application can act upon such exception.

DataStoreUnavailableException will help external applications to retry if they see this exception.

Change-Id: Iceb10580bbe73ae91dc8abb4bc6a183cb4fad6f8
Signed-off-by: Harman Singh <harmasin@cisco.com>
(cherry picked from commit 6823ef1fd8f0a2f7ea39a2e85276a73e5de0bf72)