DDL no longer aborts mysql_lock_tables(), and hence
we no longer need to support need_reopen flag of this
call. 
Remove the flag, and all the code in the server
that was responsible for handling the case when
it was set. This allowed to simplify: 
open_and_lock_tables_derived(), the delayed thread,
multi-update.

Rename MYSQL_LOCK_IGNORE_FLUSH to MYSQL_OPEN_IGNORE_FLUSH,
since we now only support this flag in open_table().

Rename MYSQL_LOCK_PERF_SCHEMA to MYSQL_LOCK_LOG_TABLE,
to avoid confusion.

Move the wait for the global read lock for cases
when we do updates in SELECT f1() or DO (UPDATE) to 
open_table() from mysql_lock_tables(). When waiting
for the read lock, we could raise need_reopen flag,
which is no longer present in mysql_lock_tables().
Since the block responsible for waiting for GRL
was moved, MYSQL_LOCK_IGNORE_GLOBAL_READ_LOCK
was renamed to MYSQL_OPEN_IGNORE_GLOBAL_READ_LOCK.