If a transaction is replayed, queued commands must not be processed. The
exception to this rule is when pending session commands are executed
before the first statement in the replayed transaction is executed.
If transaction replaying was enabled and a result was returned in more
than one call to clientReply, a NULL value would be added to the statement
which in turn would trigger a debug assertion.
Similarly any following statements in the transaction would be executed
regardless of whether the result was complete.
Renamed the statement execution function to better describe what it does.
Extended the basic functional test case to cover this.
Handle the problematic transaction with session command as well as empty
transactions. Also changed test to use wait_for_monitor as well as pass
the value to check as a parameter to the `check` function.
