class Download

When we want to download symbols for a build ID, we create a Download object. This object is

owned by the DownloadManager. We then fire off requests to all symbol servers we know about

asking to download the symbols. These requests are completed in sequence to all of the servers

the DownloadManager knows about, until one reports that the download succeeded. If all the

servers are tried and none of them are informed the request was successful, DownloadManager will

drop the object. The destructor of the Download object calls a callback that handles notifying

the rest of the system of those results.

If we receive more notifications that other servers are ready to receive download requests, they

are queued and will be tried as a fallback until the download succeeds or we run out of servers.