Remove unused import in test
Simplify tar upload pool concurrency control

Instead of relying on somewhat bizarre gevent.Group mechanics, just
maintain a counter (in the form of concurrency_burden).

This requires moving the uncharging mechanics out of _uncharge
(renamed _finish) into _wait (renamed from _get) to avoid race
conditions.

A couple of extra stress tests have been added.

The pre-patch of this code may well harbor a bug; the inspiration to
write this was encountering synchronization bugs (dangerous ones, that
could result in the silent passage of failed uploads) in an
implementation derived from this one for the purpose of parallel WAL
upload.
Amend both code and comment in cmd.py for wal-push/wal-fetch

In addition to having been an outright falsehood (wal-push and
wal-fetch comments were swapped) the wal-fetch operator was
confusingly spread out across the body of the function.
Change help text for --pool-size parameter

This referred specifically to downloads, but as a convenience of
mechanism the exact same setting is used for uploads, also.

Instead of teaching everything two help texts, unify, as it seems
clear enough.
Call out an important concurrency control invariant in TarUploadPool

The reordering here doesn't fix any bugs, but it'd be very easy to
turn this into a subtle mistake, so call it out and move the
concurrency control as close to the critical operation as possible.
Draft patch