Allow cross-db DMLs in the parser.

Changed parser to allow cross-db DMLs like:
UPDATE a.b...
These constructs will always become PASS_DML plans, which
will eventually be blocked on execution to protect update stream.
We need to allow the syntax to make sure that the rowcache invalidator
can parse and ignore such spurious DMLs introduced by
a non-vttablet process, but which don't affect rowcache.