Reduce number of syscalls in PostUploadPack and PackObjectsHook
This MR adds a number of small improvements to reduce the number of system calls used for transmitting PostUploadPack and PackObjectsHook data. This was inspired by gitlab-com/gl-infra/scalability#1024 (closed)
- Buffer response writes in PostUploadPack
- Use buffering to minimize the number of reads and writes (which happen in lockstep) in PackObjectsHook