Add internal/streamcache package

gitlab-com/gl-infra/scalability#869

This MR adds a new internal package internal/streamcache that we want to plug into the PackObjectsHook RPC in a later MR. In this MR, we are just adding library code that is not called yet outside of tests.

The intended use is short-term buffering of packfile data to disk during Git clones / fetches. However, the code should also be useable for other RPC's that can generate highly concurrent large streams of data such as GetArchive.