Use streamcache in PackObjectsHook with hard-coded configuration

gitlab-com/gl-infra/scalability#918 (closed)

This MR changes the behavior of PackObjectsHook so that it uses the cache introduced by !3106 (merged). The goal is to make just enough changes to allow us to run useful experiments in production, but nothing more.

PackObjectsHook is behind a feature flag gitaly_upload_pack_gitaly_hooks which is off by default. This MR takes advantage of that by using hard-coded configuration values for now. These values have been chosen in the expectation / hope that they work on gitlab.com, based on data collected from the previous iteration of PackObjectsHook. gitlab-com/gl-infra/scalability#872 (closed)

Depending on the outcome of our experiments, we will then either make the hard-coded values configurable, or iterate more on the design of the cache.