ALTER TABLE RENAME, DISABLE KEYS.

The code of ALTER TABLE RENAME, DISABLE KEYS could
issue a commit while holding LOCK_open mutex.
This is a regression introduced by the fix for
Bug 54453.
This failed an assert guarding us against a potential
deadlock with connections trying to execute
FLUSH TABLES WITH READ LOCK.

The fix is to move acquisition of LOCK_open outside
the section that issues ha_autocommit_or_rollback().
LOCK_open is taken to protect against concurrent
operations with .frms and the table definition
cache, and doesn't need to cover the call to commit.

A test case added to innodb_mysql.test.

The patch is to be null-merged to 5.5, which
already has 54453 null-merged to it.

mysql-test/suite/innodb/r/innodb_mysql.result:
  Added test results for test for bug#56619.
mysql-test/suite/innodb/t/innodb_mysql.test:
  Added test for bug#56619.
sql/sql_table.cc:
  mysql_alter_table() modified: moved acquisition of LOCK_open
  after call to ha_autocommit_or_rollback.