Problem:
========
 There is a possibility that there can be more concurrent DMLs While the
alter table thread is waiting for upgrading to MDL_EXCLUSIVE before commit phase.
In commit phase, InnoDB acquires dict_operation_lock and it already holds MDL_EXCLUSIVE
on the table. After that, InnoDB applies the concurrent DML logs in commit phase.
This could lead to blocking of the following things:

  1) DML on the particular table (due to MDL_EXCLUSIVE on the table)
  2) InnoDB DDLs (due to dict_operation_lock)
  3) Purge thread, stats thread, the master thread (due to dict_operation_lock)

Fix:
====
Apply the concurrent DML logs in commit phase but before acquiring
dict_operation_lock in commit phase. It makes sure that (2), (3) can't be
blocked for longer time.