(#577) - Initial implementation of auto-compaction

Right now its disabled by default and you have to enable it when creating a Pouch. However, after enabling it all our tests still pass, so it seams quite safe to do so. @daleharvey, what's your opinion?

Also, we could let users decide on the "level of compaction", namely how old revision shall be deleted. But I'm not sure if it is a good idea -- the default level=1 seams quite reasonable.