migrate `hostdb` to the from `weightedlist` to the new `hosttree` package
This PR removes weightedlist
from the hostdb
and replaces it with the new hosttree
package. Hosts are still stored in the activeHosts
map by the NetAddress, since changing fully to public keys requires changing the HostDB
interface, which is a much larger change. A subsequent PR will be opened, likely after the renter pools branch is merged, which removes activeHosts
and changes the HostDB to use public keys.