Fixes#75
## Why
On a fetch-lock miss, `Engine.Fetch` slept a flat 500ms once, tried the store, and otherwise fell through to fetch upstream unlocked. A cold-cache stampede therefore still hit upstream once per waiter.
## Changes
- Add `waitForStore`, which polls the store every 100ms for up to 5s (stopping on context cancellation) so waiters pick up the lock leaders populated result.
- Only fall through to an upstream fetch if the leader has not populated the store within the wait budget.
## Validation
- `make e2e` passes.
Reviewed-on: #93
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
unkinben
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