catfile: Use same process list when using `--batch-command`

There are two types of catfile caches present, one for ObjectReader and one for ObjectInfoReader. When we introduced --batch-command, we didn't separate the caches for the old and new implementations. Thus, the newer --batch-command implementation, uses the same cache as the old implementation.

This is a bug, since reverting the flag wouldn't switch to old implementation immediately since cache entries would still serve the newer implementation. There is also a bug/improvement wherein --batch-command can actually share the cache entry for both info/content retrieval. Let's fix this by introducing and using a new process list called objectReaders.

Eventually we can merge the implementation's into one and users can then simply request content and info together. But this requires more changes, specifically on the queue, wherein, we'd need to expect that users can request info/content interchangeably.

Part of #4573 (closed)