Remove the possibility for merge and open to race.

As described in #95, if a vnode dies and takes down your bitcask instance, it's possible that a merge will get there first, causing repeated open failures that can take the whole node down.  This change alters how open and merge work, disallowing merges if the bitcask instances is not currently opened by a pid in the same VM.

Additionally, as described in #149, initialization can hang on startup if the deletion queue is being blocked.  Since that polling was intended to fix an issue on the merge init path (which no longer exists), we can remove it.  The removal requires making some other pathways more robust.

This PR also contains some test cleanup.