Fix for bug #51263 "Deadlock between transactional

SELECT and ALTER TABLE ...  REBUILD PARTITION".

ALTER TABLE on InnoDB table (including partitioned tables)
acquired exclusive locks on rows of table being altered.
In cases when there was concurrent transaction which did
locking reads from this table this sometimes led to a
deadlock which was not detected by MDL subsystem nor by
InnoDB engine (and was reported only after exceeding
innodb_lock_wait_timeout).

This problem stemmed from the fact that ALTER TABLE acquired
TL_WRITE_ALLOW_READ lock on table being altered. This lock
was interpreted as a write lock and thus for table being
altered handler::external_lock() method was called with
F_WRLCK as an argument. As result InnoDB engine treated
ALTER TABLE as an operation which is going to change data
and acquired LOCK_X locks on rows being read from old
version of table.

In case when there was a transaction which already acquired
SR metadata lock on table and some LOCK_S locks on its rows
(e.g. by using it in subquery of DML statement) concurrent
ALTER TABLE was blocked at the moment when it tried to
acquire LOCK_X lock before reading one of these rows.
The transaction's attempt to acquire SW metadata lock on
table being altered led to deadlock, since it had to wait
for ALTER TABLE to release SNW lock. This deadlock was not
detected and got resolved only after timeout expiring
because waiting were happening in two different subsystems.

Similar deadlocks could have occured in other situations.
This patch tries to solve the problem by changing ALTER TABLE
implementation to use TL_READ_NO_INSERT lock instead of
TL_WRITE_ALLOW_READ. After this step handler::external_lock()
is called with F_RDLCK as an argument and InnoDB engine
correctly interprets ALTER TABLE as operation which only
reads data from original version of table. Thanks to this
ALTER TABLE acquires only LOCK_S locks on rows it reads.
This, in its turn, causes inter-subsystem deadlocks to go
away, as all potential lock conflicts and thus deadlocks will
be limited to metadata locking subsystem:

- When ALTER TABLE reads rows from table being altered it
  can't encounter any locks which conflict with LOCK_S row
  locks. There should be no concurrent transactions holding
  LOCK_X row locks. Such a transaction should have been
  acquired SW metadata lock on table first which would have
  conflicted with ALTER's SNW lock.
- Vice versa, when DML which runs concurrently with ALTER
  TABLE tries to lock row it should be requesting only LOCK_S
  lock which is compatible with locks acquired by ALTER,
  as otherwise such DML must own an SW metadata lock on table
  which would be incompatible with ALTER's SNW lock.

sql/sql_trigger.cc

@@ -489,7 +489,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
   else
   {
     tables->table= open_n_lock_single_table(thd, tables,
-                                            TL_WRITE_ALLOW_READ, 0);
+                                            TL_READ_NO_INSERT, 0);
     if (! tables->table)
       goto end;
     tables->table->use_all_columns();