This entails:

1. Converting the pushdowns implemented by DocumentSource::optimizeAt() to high-priority rules
2. Registering DocumentSource::doOptimizeAt() as an explicit, lower-priority rule that gets called on every document source (to be replaced by SERVER-110107)
3. Replacing implementation of optimizeContainer with a call to RBR
4. (Possibly) migrating relevant unit tests to the rbr directory