MDEV-14831 CREATE OR REPLACE SEQUENCE under LOCK TABLE corrupts the
sequence, causes ER_KEY_NOT_FOUND

The problem was that sequence_insert didn't properly handle the case
where there where there was a LOCK TABLE while creating the sequence.

Fixed by opening the sequence table, for inserting the first record, in
a new environment without any other open tables.

Found also a bug in Locked_tables_list::reopen_tables() where the lock
structure for the new tables was allocated in THD::mem_root, which causes
crashes. This could cause problems with other create tables done under
LOCK TABLES.