Fix read-only abort message mismatch

Fix read-only abort message mismatch

Testing has revealed:

WARN  | FrontendReadOnlyTransaction | Rejecting unsupported request ModifyTransactionRequest{target=member-2-datastore-config-fe-0-txn-2-0, sequence=1, replyTo=Actor[akka.tcp://opendaylight-cluster-data@10.29.15.184:2550/user/$a#585956314], operations=[], protocol=ABORT}

This is a thinko on the part of which message does what:

TransactionAbortRequest is dedicated for 3PC doAbort phase, hence
it is never seen for read-only transactions.

The message corresponding to an abort is either
AbortLocalTransactionRequest or ModifyTransactionRequest with protocol
set to ABORT.

Change-Id: I3238ade7b9f7933e6538742354888d182f599412
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>