12447: Reduce memory footprint of crunch-run.
- BlockCache reads into a fixed-capacity buffer, to avoid reallocations (this
cuts resident memory usage by 1/2 and makes memory footprint extremely
- Limit the crunch-run BlockCache to 2 blocks instead of 4 (cuts footprint by
- Clear the crunch-run BlockCache after loading the Docker image (probably cuts
post-image-load footprint by 1/2 again).
Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <email@example.com>