Fixed failure in mtr --ps sql_sequence.create sql_sequence.read_only
Fixed failure in mtr --ps sql_sequence.create sql_sequence.read_only

Problem was that we got an error in sequence_insert while opening the
newly created sequence table in an prepared statement as the table id
didn't match.
Fixed by temporarly removing the reprepare observer during
sequence_insert as there can never be a table missmatch in this case.

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2