ParserStatus is now more lightweight, and doesn't know about "result"
and "from cache". Instead, it has an isValid() method which is merely a
shorthand for checking whether getException() is null.
Introduce a child class, RuleCheckerStatus, which knows about result and
cache and can be (un)serialized.
This removes the ambiguity of the $result field, and helps the
transition to a new RuleChecker class.
Change-Id: I0dac7ab4febbfdabe72596631db630411d967ab5
