v0.13.0

Lucky release number 13 brings some reasonably big changes:

• Big performance boost for maxwell: 75% faster in some benchmarks
• @davidsheldon contributed some nice bug fixes around CREATE TABLE ... IF NOT EXISTS, which were previously generating new, bogus copies of the schema.
• we now include a "scavenger thread" that will lazily clean out old, deleted schemas.

This release has a pretty bad bug. do not use.