XA Transactions

Summary:
This WL creates new backup meachnism to hold MDL locks when
there is no active THD object associated with it. The following
use cases are covered by this new mdl backup subsystem.

When a client connection is broken/ends after making a
PREPARED XA transaction, transaction goes into detached
state and stays in XA transaction cache. MDL locks associated
with such transactions are transferred to MDL backup subsystem.
Backedup MDL locks will be freed once the transaction is
finished from another connection by subsequent XA COMMIT or
XA ROLLBACK.

When server goes down(graceful or crash) after XA transaction
moved to PREPARED state, Innodb recovers such XA transactions during
server start and puts the transaction in server's XA transaction
cache. Relevant MDL locks are taken during the same time and added
to MDL backup subsystem to protect participating table(s) from DDL(s).
Backedup MDL locks will be freed once the transaction is finished from
another connection by subsequent XA COMMIT or XA ROLLBACK.

Additionally, refactoring was done as part of worklog implementation to
extract code used for commit/roll back of prepared XA transaction
into separate methods (extract method refactoring). Both branches used for
handling of external and internal transaction branches were refactored.