On Windows s3d was stopped with TerminateProcess. One killed during an
upload (a Theseus restart, a cut-off flush) had deleted four buffered
files without recording them as uploaded; every flush afterwards failed
on them and the files were gone.
A stop now waits while a flush Pithos started is running, or while s3d
has begun an upload batch in the last two minutes (up to five minutes;
twenty seconds when the host is quitting), then asks s3d to shut down
cleanly: Ctrl+Break sent to its own hidden console on Windows (Windows
disables Ctrl+C in a new process group; Go reads both as an interrupt),
SIGTERM elsewhere. It is ended by force only if still there a minute
later. Checked against s3d v0.2.0: 'shutdown complete', exit code 0.
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