BUG-650: speedup DOMDataBrokerTransactionChainImpl

The transaction chain implementation does not need to have
synchronization on submit, but rather it needs to check if the chain has
not been closed (implictly or explicitly). Same goes for the close path,
thus this patch speeds things up by downgrading the synchronized
sections to volatile reads and writes.

Change-Id: I57dec6dbfb1933005a9ee0718b666ee102ce7116
Signed-off-by: Robert Varga <rovarga@cisco.com>