MDEV-17772 - 3 way lock : ALTER, MDL, BACKUP STAGE BLOCK_DDL

While waiting for a (potentially long) RO transaction or SELECT, DDL and
LOCK TABLES ... WRITE hold protection against FTWRL and BACKUP STAGE.

This effectively makes FTWRL/BACKUP STAGE indirectly wait for this RO
transaction or SELECT to finish. Which is not great, as otherwise we
could do something useful meanwhile.

With this patch BACKUP lock is attempted to be acquired after TABLE/SCHEMA
locks. If this attempt fails, TABLE/SCHEMA locks gets released and we
start waiting for BACKUP lock. When wait finishes, BACKUP lock is released
(to avoid deadlocks) and we attempt to acquire all locks once again.

Other changes:
- Take MDL lock before testing if table exists as part of
  CREATE TABLE ... IF EXISTS.  This change was an effect of changes in
  lock_table_name and removes an inconsistency where one could get
  different error messages from CREATE TABLE .. IF EXISTS depending on
  active mdl locks.
  One effect of this change is that we don't binary log CREATE TABLE IF
  EXISTS if the table exists.  This was done because old code was sometimes
  behaving inconsistenly (it was logged some time and not other times)
  and sending the query to the slave could make the slave even more
  inconsistent as there is not guarantee that the new table will have
  the same definition as the old table on the master.