Oops... otherwise we crash.

This is now done in send_request.

This makes sure that any neighbour answering the request has up-to-date
information about us.

We only expire sources every 5 minutes, since it's a somewhat costly
operation.

Now flush_source itself checks for the source being flushable.

Since this is a rather costly operation, we only do it when we overflow
the source table.

Instead of neigh->id[0] = 0xFF.