holds_original, the prefetch worker's check that a neighbour's original
is already cached, opened the catalog for every neighbour it asked about.
Its own comment called it a row check; the open around it was four of
the five opens a develop landing made.
The worker now keeps one Catalog for the batch it is serving, opened at
the first check and reopened only if the batch names another catalog
file. The connection runs in autocommit, so each check still sees what
fetch_original committed in between. fetch_original is unchanged.
With the backfill no longer run on every open, a landing whose
neighbours are all cached goes from five opens to two, and from ~80 ms
of CPU to ~1-2 ms on a copy of the reference catalog.
dtourolle
faf52f6dbd