Waiters that lost the refresh lock polled the shared cache for only 750ms
before falling through to the upstream, but the refresh lock lives for 15s.
On a cold cache with an upstream slower than 750ms, every waiting pod gave up
and stampeded the origin (each also writing L2). Extend the poll window to
match the lock TTL so waiters return the entry the holder publishes instead.
Adds a regression test that fails without the fix.
Thomas Camlong
0ca62f1232