FLUSH LOGS should ignore SET GLOBAL READ_ONLY.


sql/lock.cc:
  Sdditional fix-up for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - ignore SET GLOBAL READ_ONLY if MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY is set.
sql/mysql_priv.h:
  Sdditional fix-up for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY added.
sql/sql_base.cc:
  Sdditional fix-up for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - ignore SET GLOBAL READ_ONLY as well if called form a logger.