One concurrent download per host for all hosts

[HebaruSan]

Background

In #3054 we changed the async downloader to sequentialize downloads from GitHub to avoid triggering GitHub's download throttling. Only one GitHub download will ever be active concurrently, and this seems to have solved the problems we were having with throttling.

Problems

• I have been installing big groups of mods to test Detect manually installed mods via cfg file :FOR[identifier] clauses #3525, and I've found that even SpaceDock does not seem to like having a lot of parallel downloads; any beyond 2 or 3 seemed to fail and fallback to archive.org or require retrying the installation. [Feature] Download Limits #3314 may be a report of this same problem, if the hosts of the affected downloads were mainly SpaceDock.
• In addition, Mono.Unix.UnixIOException can be thrown if there are many concurrent downloads in progress (see [Bug][Solved] Mono.Unix.UnixIOException: Resource temporarily unavailable on Arch Linux after Util.Invoke and long chain of runtime calls  #3343 and Download fails and crashes ckan with FATAL UNHANDLED EXCEPTION (Mono.Unix.UnixIOException: Resource temporarily unavailable [EWOULDBLOCK]) #3555).

Causes

• It's hard to know for sure, but I suspect that the first few downloads from SpaceDock quickly fill the pipe, and then the remaining ones either fail to start due to congestion or are deliberately throttled by the server.
• Too many UI updates at once can overwhelm Mono. A fix for this was submitted in Fix unhandled UnixIOException in XplatUIX11 mono/mono#21136, but it shows no signs of having been looked at by the Mono team in the last 9 months, if such a team still exists. CKAN can be affected by this Mono bug if there are many concurrent downloads, because each of them has its own progress bar that we update every time another chunk of the download arrives.

Changes

Now the single concurrent download behavior is applied to all download hosts, per host. When you start a large group of downloads, we will start one from each host in the group (GitHub, SpaceDock, etc.) in parallel, and as each download finishes the next one from that host will be started, until they're all done. This should fix problems arising from too many downloads from one host.

Fixes #3314 (probably, depending on which downloads were failing for that user).
Fixes #3555 (hopefully, by reducing the number of GUI updates during downloads).

[DasSkelett]

Works like a charm, let's hope they do resolve some of the download issues we've seen.