p2phttp: notice when new repositories are added to --directory

p2phttp: notice when new repositories are added to --directory

When a uuid is not known, rescan for new repositories. Easy.

When a repository is removed, it will also get removed from the server
state on the next scan. But until a new uuid is seen, there will not be
a scan. This leaves the server trying to serve a uuid whose repository
is gone. That seems buggy. While getting just fails, dropping fails the
first time, but seems to leave the server in an unusable state, so the
next drop attempt hangs. The server is still able to serve other uuids,
only the one whose repository was removed has that problem.