proxy: download packages concurrently

[shoenig]

Currently the proxy has a singe background.Reloader goroutine, which every 30 seconds or so queries the registry for a complete list of modules, and then either ignores (already have it) or downloads each module from the upstream source one at a time. Instead, every iteration when it's time to download modules, spin up a configurable pool of worker threads per-domain which can download more than one module at a time from that upstream domain. That way, even if we want to limit ourselves to downloading from 1 VCS at a time, we're still downloading N modules at a time, 1 from each of the VCSs that appear in the list of modules to keep.