BUG-6111: implement PingPongTransactionChain cancelation
This patch implements transaction cancelation in PingPongDataBroker,
which has slightly different semantics -- if a transaction is canceled
while being in a batch, proper isolation of the batch is maintained
and after preceding batch completes, the transaction chain is aborted.
Since there is no transaction isolation within a batch, this is the
only course of action we can take.
Change-Id: I0058503165dbfba8748a17a9ef9272265f4bc1c9
Signed-off-by: Robert Varga <rovarga@cisco.com>