Bug 6540: EOS - handle edge case with pruning pending owner change commits

Bug 6540: EOS - handle edge case with pruning pending owner change commits

The previous patch https://git.opendaylight.org/gerrit/#/c/45516/ added
pruning of pending owner change commits on leader change. However there's
one edge case which wouldn't work correctly where the leader successfully
commits a transaction to add a candidate but becomes isolated when it tries
to commit the transaction to set the candidate as the owner, assuming the new
candidate is the only candidate. When the partition is healed, the owner write
transaction will be pruned and dropped thus no onwer will be selected.

We could allow this owner write to be forwarded to the new leader since it
originated from a client candidate add request. However this could still be
problematic if, during isolation, the majority partition gets a candidate add
and commits an owner. After the partition heals the "old" owner write would be
forwarded and overwrite the previous owner. This wouldn't be catastrophic but
would incur an unnecessary owner change. I would rather keep consistent behavior
of dropping pending owner writes to a new leader.

Instead, the new leader can assign the previous leader as owner when the partition
heals. So in onPeerUp and onLeaderChange, I added code to search for all entities
with no owner and select and write an owner. Therefore when onPeerUp occurs for the
previous leader after isolation, if no other candidate was registered and became
owner, then the previous leader will be assigned as owner.

Change-Id: I213bc3ecd3d1f7ebd099702390de2277109f92c2
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>
(cherry picked from commit 0fab6c716548e89938c1a8493dc25991c006aa10)