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.