ipcache is not fully threadsafe

[pboothe]

It is designed to be used like:

if !cache.Has(ip) {
  cache.Add(ip)
  Trace(ip)
}

but this check-then-add pattern is not threadsafe. Instead, we should use an unconditional Add that returns true if the item was not previously in the cache. Then the usage would be:

if cache.Add(ip) {
  Trace(ip)
}

which (as long as Add is mutex-protected) is threadsafe.

[pboothe]

Ya, I assigned this one to you because you will be moving the location of the cache around already, so it seems like a small bug to fix as part of that work.