MDEV-10139 Support for SEQUENCE objects

- SETVAL(sequence_name, next_value, is_used, round)
- ALTER SEQUENCE, including RESTART WITH

Other things:
- Added handler::extra() option HA_EXTRA_PREPARE_FOR_ALTER_TABLE to signal
  ha_sequence() that it should allow write_row statments.
- ALTER ONLINE TABLE now works with SEQUENCE:s

sql/sql_cmd.h

@@ -98,6 +98,7 @@ enum enum_sql_command {
   SQLCOM_EXECUTE_IMMEDIATE,
   SQLCOM_CREATE_SEQUENCE,
   SQLCOM_DROP_SEQUENCE,
+  SQLCOM_ALTER_SEQUENCE,
 
   /*
     When a command is added here, be sure it's also added in mysqld.cc