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.
initial postgres backend implementation. very slow
move transactions into the iterator to make cursors more stable/reliable
adding LRU and deferred constraints for loading
new schemas, data structs, and iterators, oh my!

making "provenance" a node type instead of a varchar in DB
adding data to TSVals for triples per discussion on google/cayley#25
splitting iterator implementations to reduce overhead/typecasts
bring in line with upstream
fix copy-paste bug
update for new interfaces