Finally have all the groundwork done for canceling stalled transfers. This
involved taking some code that was in the assistant, and had not been
touched for probably 7 years beyond basic maintenance, dusting it off,
and making it suitable to be used in git-annex more generally. Now I have
git-annex using transferkeys child processes, and all that seems to work
well.

I'm finishing up today by designing the new git config that will enable stall
detection and canceling. annex.stalldetection will be configurable to a
value like "1MB/30s", which means it's stalled unless every 30 seconds
a megabyte of data has been tranferred. Or "0KiB/2m" will let things stall
for up to 2 minutes with no data transfer. There will also be a per-remote
config, so minimum transfer rates can be set for each. This can be combined
with annex.retry to make it retry after detecting a stall.

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Today's work was sponsored by Jake Vosloo
[on Patreon](https://patreon.com/joeyh).