Bug 4627: Fix premature RO tx cleanup

Bug 4627: Fix premature RO tx cleanup

For the RO tx PhantomReference cleanup mechanism, modified
RemoteTransactionContextSupport to pass the front-end client
TransactionProxy instance as the referent to the
FinalizablePhantomReference. Previously we were passing the
RemoteTransactionContextSupport instance which is only reachable via
a hard reference until the primary shard actor is obtained and thus may
be eligible for GC while the TransactionProxy is still in use.

Change-Id: Ib2808b4ba8113a5722f9ee422434a89adaf775fe
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>