The log tables are by nature PERFORMANCE_SCHEMA tables,
which should not be affected by SET GLOBAL READ_ONLY or FLUSH TABLES
WITH READ LOCK.

The implementation of FLUSH TABLES WITH READ LOCK already ignored log tables.
Now with this patch, the implementation of LOCK TABLE also ignore a
global read lock for log tables, which was the missing symmetry.


mysql-test/r/flush.result:
  Fix for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - test result.
mysql-test/t/flush.test:
  Fix for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - test case.
sql/lock.cc:
  Fix for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - logger.is_privileged_thread() used.
sql/log.h:
  Fix for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - LOGGER::is_privileged_thread() introduced that returns TRUE if a given thread 
      is either a general_log or a slow_log or a privileged thread.
sql/sql_base.cc:
  Fix for bug #26380: LOCK TABLES + FLUSH LOGS causes deadlock
    - pass MYSQL_LOCK_IGNORE_GLOBAL_READ_LOCK to the mysql_lock_tables() in case of 
      call from a logger in order not to honor the GLOBAL READ LOCK 
      and to avoid possible deadlocks.