Explore an storage abstraction layer using iterator trees

As part of the effort to enable Cayley (http://github.com/google/cayley) as a backend for BadWolf, its efficient usage would require to provide an iterator tree as an output of the BQL parsing for Cayley to pick it up. This would enable Cayley to use the optimizer before executing the query. The iterator tree should be translatable to the final Cayley one as discussed with @barakmich.