Bug 4774: Wait for prior RO tx creates on tx chain

Bug 4774: Wait for prior RO tx creates on tx chain

Added a priorReadOnlyTxPromises map to TransactionChainProxy that holds
Promise instances for each read-only tx. When the parent class completes
the primary shard lookup and creates the TransactionContext (either success or
failure), onTransactionContextCreated is called which completes the Promise. A
write tx that is created prior to completion will wait on the Promise's Future via
findPrimaryShard.

Change-Id: Ib1a620cfd5be3e38f633b3faf9ef7a31abaaf345
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>