Hostdb debugging

As this is a debugging PR, the commits are all over the place because I was poking around and tuning/tweaking.

generally speaking, I added more/better logging, including a String() method for types.SiaPublicKey. I changed the weird 10,000 gothread queue into a slice-based queue, which should make life easier for the scheduler. I swapped out some potentially buggy height update code for some known height update code. I added some shutdown statements so we can see what's going wrong when siad is taking forever to shutdown. Thus far it seems to be the host.

I don't think I solved any real problems here, other than adding more logging which allows us to better see what's going wrong if a host cannot be contacted.