No test case, since the bug requires a stress case with 30 INSERT DELAYED
threads and 1 killer thread to repeat. The patch is verified
manually.
Review fixes.

The server that is running DELAYED inserts would deadlock itself
or crash under high load if some of the delayed threads were KILLed
in the meanwhile.

The fix is to change internal lock acquisition order of delayed inserts
subsystem and to ensure that
Delayed_insert::table_list::db does not point to volatile memory in some 
cases.
For details, please see a comment for sql_insert.cc.


sql/sql_insert.cc:
  A fix for Bug#29431 killing an insert delayed thread causes crash
  
  1) The deadlock was caused by different lock acquisition order
  between delayed_get_table and handle_delayed_insert.
  
  delayed_get_table would:
  - acquire LOCK_delayed_create
  - create a new Delayed_insert instance
  - acquire instance mutex (di->mutex)
  - "lock the instance in memory" by 
  increasing di->locked_in_memory variable
  - start the delayed thread
  - release di->mutex
  - let the delayed thread open the delayed table
  - discover that the delayed thread was killed
  - try to unlock() the delayed insert instance in memory
  - in Delayed_insert::unlock() do
   * lock LOCK_delayed_insert
   * decrease locks_in_memory and discover it's 0
   * attempt to lock di->mutex to broadcast di->cond condition <-- deadlock
   here
  
  Meanwhile, the delayed thread would
   * lock di->mutex
   * open the table
   * get killed
   * not notice that and attempt to lock LOCK_delayed_insert
  to register itself in the delayed insert list <-- deadlock here.
  
  Simply put, delayed_get_table used to lock LOCK_delayed_insert and then 
  di->mutex, and handle_delayed_insert would lock di->mutex and then 
  LOCK_delayed_insert.
  
  Fixed by moving registration in the list of delayed insert threads from 
  handle_delayed_insert to delayed_get_table - so that now
  handle_delayed_insert doesn't need to acquire LOCK_delayed_insert mutex.
  
  
  2) di->table_list.db was copied by-pointer from table_list.db of the first
  producer -- the one who initiated creation of the delayed insert thread.
  This producer might be long gone when the member is needed
  (handle_delayed_insert:open_ltable),
  e.g. by having been killed with KILL CONNECTION statement.
  Fixed by using a pointer to the consumer's deep copy of THD::db.
  
  3) In find_handler, we shouldn't assume that Delayed_insert object
  already (or still) has a table opened all the time it is
  present in the delayed insert list. E.g. it's not the case
  when Delayed_insert decided to terminate, closed the table, but haven't
  yet unregistered from the list (see the end of handle_delayed_insert).