Handle the 'set read_only=1' in lighter way, than the FLUSH TABLES READ LOCK;
    For the transactional engines we don't wait for operations on that tables to finish.

per-file comments:
 mysql-test/r/read_only_innodb.result
MDEV-136 Non-blocking "set read_only".
       test result updated.
 mysql-test/t/read_only_innodb.test
MDEV-136 Non-blocking "set read_only".
       test case added.
  sql/mysql_priv.h
MDEV-136 Non-blocking "set read_only".
        The close_cached_tables_set_readonly() declared.
  sql/set_var.cc
MDEV-136 Non-blocking "set read_only".
         Call close_cached_tables_set_readonly() for the read_only::set_var.
   sql/sql_base.cc
 MDEV-136 Non-blocking "set read_only".
         Parameters added to the close_cached_tables implementation,
         close_cached_tables_set_readonly declared.
         Prevent blocking on the transactional tables if the
         set_readonly_mode is on.