Fix TODO in PickPeer

Use a map to look up the httpGetter, rather than allocating every time.
Add Remove capability

Group now has a Remove() method. This method attempts to remove the
entry from the local cache. If the local cache does not have the entry,
it calls out to the appropriate peer using a DELETE method, which
triggers a remove on the peer.

This change does not remove the key from the hot cache, nor does it
instruct all peers to remove it from the hot cache. Hot cache handling
with remove capability needs further investigation and work.

This change is fully in-memory tested. Actual testing with live servers
is yet to be done.