Bug 6587: Retain state when transitioning between Leader and IsolatedLeader

Bug 6587: Retain state when transitioning between Leader and IsolatedLeader

If there's a transaction in the COMMIT_PENDING state, ie it has been persisted and
is in the process of being replicated, and the Leader switches to IsolatedLeader, the
ClientRequestTracker state is lost. As a result when the follower(s) come back and
replication consensus is achieved and the tx is applied to state, the tx ID isn't
available and the ShardDataTree applies it as a foreign candidate, leaving the
tx in the pending queue. This prevents subsequent transactions from making progress.

To fix this, we need to retain/copy the internal leader state when transitioning
between Leader and IsolatedLeader.

Change-Id: If06996dccf083fd5d37757fd91fde2eb0eb82ea1
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>