Proper Recovery for BdbDB backed stores

[tmagrino]

Right now we have some code written in the hopes we can restart a crashed or stopped store backed by BdbDB but it's not clear when or how we could use this feature.

There should probably be a version of BdbDB that has similar fault tolerance to MemoryDB (persists if stopped gracefully, otherwise no real guarantee) that allows for spilling to disk and a separate version for enhanced failure recovery.