Added protection against global read lock while creating and
initializing a delayed insert handler.
Allowed to ignore a global read lock when locking the table
inside the delayed insert handler.
Added some minor improvements.


sql/lock.cc:
  Bug#7823 - FLUSH TABLES WITH READ LOCK + INSERT DELAYED = deadlock
  Changed mysql_lock_tables() to allow for ignoring global read lock.
  Added functions to set/unset protection against global read lock.
sql/mysql_priv.h:
  Bug#7823 - FLUSH TABLES WITH READ LOCK + INSERT DELAYED = deadlock
  Changed existing and added new function declarations.
sql/sql_insert.cc:
  Bug#7823 - FLUSH TABLES WITH READ LOCK + INSERT DELAYED = deadlock
  Added and extended some comments.
  Added a protection against global read lock while a handler is
  created and initialized.
  Moved the unlock of the delayed insert object past its last usage
  in delayed_get_table().
  Changed the table locking in handle_delayed_insert() so that it
  does not wait for global read lock.