Register should be RegisterIterator
move to registry interface for backends
update names per discussion at google/cayley#38
adding to A+C
fix tests for new method names
6 of one, half-dozen of the other
fix failing build
comment tweaks
Merge pull request #69 from pbnjay/triplestore_registry

Triplestore registry
Give TripleStore a chance to optimize Iterators
We do it after the iterators have a chance to simplify themselves first.

NB also need to make sure to Close() all unused iterators so we don't
leave stray database connections around.
Missing access to optional sub-iterator

We'll need this to optimize it later.