[BUGFIX beta] fixing broken computed.sort semantics

Conflicts:
	packages/ember-runtime/lib/computed/reduce_computed_macros.js
	packages/ember-runtime/tests/computed/reduce_computed_macros_test.js
Make computed.sort generate an answer immediately

This is a followup to pull #9356 that gives nicer timing. It extends the
arrayComputed protocol with a `flushedChanges` callback so that
implementations like `computed.sort` can confidently do batched
operations without waiting a whole run loop.