hostFetcher now refuses to fetch for any host but its own.  hostFetcher is no longer aware of Q, and uses the CmdHandler interface in place of the Handler interface.
extract Fetcher.newHostFetcher from Fetcher.handleCommand
Expose hostFetcher, is now HostFetcher.  Wrap HostFetcher and another Command channel into hostFetcherInfiniteQ.  Replace HostFetcher.Host with BaseURL.  Move some variables from hostFetcher.processChan into member fields.  Add HostFetcher.Start() which handles the robots.txt work and then calls processChan().