Fix shard commit deadlock

When performing coordinated commits we need to ensure transactions
hit all shards in the same order as otherwise we could end up
with an ABBA deadlock -- which gets resolved through commits timing
out, but nevertheless it imposes a dealy of up to 30 seconds.

Make sure we always send entries to shards in the same order and
also synchronize all TransactionProxy instances so that transactions
have a total order.

JIRA: CONTROLLER-1833
Change-Id: Ia3076efde1c7cb5f115305593776ffa7422e7a09
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>