contactmodel: look up name at most once per peer

The ContactModel::bestNameForContact function currently performs a
lookup on the name server every time it is called if its argument is the
URI of a peer which isn't a contact and doesn't have a registered name.
This patch modifies the function's behavior so that it doesn't perform
more than one lookup per peer, thus preventing the server from getting
spammed with large numbers of unnecessary requests.

GitLab: #1882
Change-Id: I7b7ef6a41d3db1001fc1418f8f35bd06b0932624