parser: support for INSERT...SET syntax

We're seeing some usage of this syntax in our binlogs that causes
the rowcache invalidator to throw false warnings. Adding it to the
grammar, but we convert the AST such that it will emit the
INSERT...VALUES representation.
invalidator: remove spammy cross-db dml warning

There's apparently going to be lots of cross-db dmls when we start
sharding. So, this warning can cause log spam.