If the starting of a transaction was interrupted by a server failure, the
query needs to be retried. This needs to be done as a transaction replay
to keep the routing logic consistent and simple.
When a non-autocommit transaction is interrupted, there will be no query
in progress and no replaying is needed. To handle this case, the replay
initialization logic needed to be altered to treat truly empty
transactions as a success case.
Added the `transaction_replay_max_size` parameter that controls the
maximum size of a transaction that can be replayed. If the limit is
exceeded, the stored statements are released thus preventing the
transaction from being replayed.
This limitation prevents accidental misuse of the transaction replaying
system when autocommit is disabled. It also allows the user to control the
amount of memory that MaxScale will use.
Added the initial implementation of transaction replay. Transactions are
only replayed if the master fails when no statement is being executed.
The validity of the replayed transaction is done by verifying that the
checksums of the returned results are equal.
Added a close function into the Trx class to make resetting its state
easier. Also changed the return type of the pop_stmt to GWBUF* as the
places where it is used expect a raw GWBUF pointer.
