#51 Periodically ping hosts, remove hits from down hosts

[Jonathan Melhuish]

To keep the search results fresh, perhaps Mutella could
ping the hosts (or, better, try to connect to their
Gnutella port) that have returned search results
periodically for a user-defined period before giving up.

Search results from hosts that did not return a pong
last time they were scanned should then be excluded
from the search results.

Cheers,

Jon