ovsdb: Use direct pointer from table to txn_table to simplify code.

ovsdb: Use direct pointer from table to txn_table to simplify code.

Until now, when a transaction modified rows in a table, the metadata
associated with that table modification (in struct ovsdb_txn_table) had to
be looked up through a hash table.  This made the code unnecessarily
complicated and had no benefit in itself, so this commit changes
struct ovsdb_table to have a direct pointer to its ovsdb_txn_table.