Global ratelimiter: improved shadow-limiter accounting

[Groxx]

Comments in code describe most of the details and reasoning, and they're in code
rather than this commit message because they need to be kept in mind when touching
this implementation.

Test changes include a "actually use reserve, oops" fix and a new test which failed
previously (shadow was counted as idle, because it had not used any tokens, because
it was canceled and returned).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.87%. Comparing base (f3350d0) to head (43a3dc5).

Additional details and impacted files

Files	Coverage Δ
...mmon/quotas/global/collection/internal/shadowed.go	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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