After the faster walk, a read that found the listing stale still took
0.5-1.0 s against 0.07 s: it was served from the old listing but ran beside
the refresh walk it had just started, sharing the GIL. The refresh now starts
REFRESH_DELAY (1 s) after that read through a per-root rewalk_handle, so the
page's own requests finish first. One pending refresh per root and none while
a walk runs; any walk start (a cold read, Rescan, the warm-up) cancels a
pending one; on_cleanup cancels them before its first await.
Vito Sansevero
a320acc41b