Removed duplicate call to handler::external_lock() when
ALTER TABLE that doesn't need to copy a table (quick
ALTER TABLE) was executed.

Also quick ALTER TABLE doesn't hold LOCK_open anymore when
it enables/disables indexes.


sql/sql_table.cc:
  Do not call handler::external_lock() as table is already locked
  by open_ltable().
  
  Also do not hold LOCK_open mutex for alter_table_manage_keys() as
  this function doesn't require LOCK_open (LOCK_open is required by
  wait_while_table_is_used() only).