INSERT DELAYED on a replication slave was converted to regular INSERT,
whereas it should try concurrent INSERT first.

With this patch we try to convert delayed insert to concurrent insert on
a replication slave. If it is impossible for some reason, we fall back to
regular insert.

No test case for this fix. I do not see anything indicating this is
regression - we behave this way since Nov 2000.


sql/sql_insert.cc:
  If we're executing INSERT DELAYED on a replication slave, we're upgrading
  lock type to TL_WRITE as we need to ensure serial execution of queries on
  the slave.
  
  OTOH if we're executing regular INSERT on a replication slave, we're
  trying TL_WRITE_CONCURRENT_INSERT first, and if we may not use it, we
  fall back to TL_WRITE.
  
  Fixed INSERT DELAYED on a replication slave to behave the same way as
  regular INSERT, that is to try TL_WRITE_CONCURRENT_INSERT first.