Delay between same-domain requests based on time when download was finished

[aecio]

Currently, the delay between requests to the same domain (host) considers only the time when the URLs were scheduled to be downloaded, not the time when the download was finished. This can cause problems when downloading from slow servers or when large files that take minutes to download. ACHE should record the time when the last download from a host was finished, and compute the next visit based on this time.