The multi-year Google Health backfill run walked three and four years of
heart rate on every pull request, some 3.7 million readings in over twenty
minutes, which filled most of its shard's time budget. What a run costs is
its pages, so a shorter history only helps with shorter slices.
HEALTHLOG_BACKFILL_TEST_DAYS now sets the size. Unset, the runs walk 120
and 160 days in 50-page slices with the faults at the same places within
them and the light collections already walked, about a tenth of the
pages. The heap growth bound applies to the full run only. The full three
and four years run nightly and on demand in integration-long.yml.
Refs #1220
Marc-André Bombeck