add hostdb subpackage `hosttree` for storing and retreiving weighted hosts

This PR introduces the hosttree subpackage to the renter's hostdb. This package exposes a minimal interface for inserting, deleting, modifying, and fetching hosts from the tree. A later PR will implement this package with the hostdb. Most of the testing from the previous weightedlist implementation has been preserved. Hosts are now indexed by Public Key instead of NetAddress.