blob: Improve latency and memory consumption for LFS pointers

The RPCs which enumerate LFS pointers all follow the same pattern: first they spawn a couple of Git commands, then they filter down objects such that we end up with LFS pointers, only, and then finally we send out the list of LFS pointers. Depending on repository size, this pattern can be quite inefficient given that we first collect all LFS pointers only to send them out at the end. This means both that we need to allocate a potentially huge chunk of blob, and that we cannot parallelize sending of LFS pointers and the Git processes.

Improve this by passing down a chunker. This allows us to easily send LFS pointers in batches while we're consuming them as soon as we hit the gRPC message size limit. As a result, we don't have to allocate a big array of LFS pointer pointers, and we can start sending while Git is still doing its thing.

Changelog: performance Fixes #3608 (closed)