Add new column 'ndb_schema_op_id' to ndb_schema table. The new column is
added when creating ndb_schema for the first time or when ndb_schema table
is upgraded. The --ndb-schema-upgrade-allowed flag can be sued to
control wheter upgrade of ndb_schema is allowed.

The new column is used for sending an id which together with nodeid will
identify the schema operation and associated resources on the node where
the schema operation is started. The column will also serves as a
marker to participants that they can start using the new
ndb_schema_result table for returning the result. This will happen
if the participant is upgraded to support the new table.

Extend NDB_SCHEMA_OBJECT with placeholders for nodeid and schema_op_id,
these wil be used by coordinator when looking up the NDB_SCHEMA_OBJECT
corresponding to the results from ndb_schema_result. Save the four "key
parts" in NDB_SCHEMA_OBJECT to be used after the reverse lookup by
schema_op_id.

Add tests which verifies that DDL operations work both with and without
ndb_schema_op_id added to ndb_schema table.

Change-Id: I5e3498b8d103dff6c40d6e09f3364934e78fa496