ovs.db.types: Add table reference to ovs.db.types.BaseType.

ovs.db.types: Add table reference to ovs.db.types.BaseType.

Until now ovs.db.types.BaseType has kept track of the name of the
referenced table but not a reference to it.  This commit renames the
ref_table attribute to ref_table_name and adds a new ref_table attribute
whose value is a reference to the named table.

This will be useful in an upcoming commit where table references are
actually followed.