Bug #16305265 HANG IN RENAME TABLE

This is a deadlock that will also be fixed in the server by
Bug #11844915 - HANG IN THDVAR MUTEX ACQUISITION.
So this is a simple alternate method of fixing the same problem,
but from within InnoDB.

The simple change is to make rename table start a transaction
before locking dict_sys->mutex since thd_supports_xa() can call
THDVAR which can lock a mutex, LOCK_global_system_variables, that
is used in the server by many other activities.  At least one of
those, sys_var::update(), can call back into InnoDB and try to
lock dict_sys->mutex while holding LOCK_global_system_variables.

The other bug fix for 11844915 eliminates the use of
LOCK_global_system_variables for calls to THDVAR.

Approved by marko in http://rb.no.oracle.com/rb/r/2000/

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