Fix PE-1584:
+ When a user txn is started, arrange to determine whether it is an XA txn or not.
  Then, correctly report whether the connection is under an XA txn when checking validity
  of DDL.  Fixes innodb_bug53674.
+ Make some more variables emulated but not passthrough per Duk's suggestion
+ Moved the error mapper into common in preparation for handling more of
  the exit points in the netty stack.