TransactionAbortRequest is used to indicate both user abort
and 3PC abort, whereas current backend code assumed it is
only used for 3PC -- hence it required a cohort to be present.
Teach handleTransactionAbort to use a direct transaction abort
if it receives a request without having a cohort present.
Change-Id: Ia469d907edb575d5f1ee5e4f630fe1a19204032f
Signed-off-by: Robert Varga <rovarga@cisco.com>