QPID-4327: HA TX transactions: basic replication.

On primary a PrimaryTxObserver observes a transaction's TxBuffer and generates
transaction events on a tx-replication-queue. On the backup a TxReplicator
receives the events and constructs a TxBuffer equivalent to the one in the
primary.

Unfinished:
- Primary does not wait for backups to prepare() before committing.
- All connected backups are assumed to be in the transaction, there are race
  conditions around brokers joining/leavinv where this assumption is invalid.
- Need more tests.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1509423 13f79535-47bb-0310-9956-ffa450edef68
QPID-4327: HA TX transactions, blocking wait for prepare

Backups send prepare messages to primary, primary delays completion of prepare
till all are prepared (or there is a failure).

This is NOT the production solution - blocking could cause a deadlock. We need
to introduce asynchronous completion of prepare without blocking. This
interim solution allows testing on other aspects of TX support.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1509424 13f79535-47bb-0310-9956-ffa450edef68