On a fetch-lock miss the engine slept a flat 500ms once and, on a store
miss, fell through to fetch upstream unlocked anyway, so a cold-cache
stampede hit upstream once per waiter. Poll the store for up to 5s (100ms
interval, context-aware) so waiters pick up the leader's result instead of
duplicating the upstream fetch.
Refs #75
unkinben
a3b972e7b1