Limit memory usage of ssh-exec during large pull operations

Summary: Fixes T4241. Ref T4206. See T4241 for a description here. Generally, when we connect a fat pipe (git-upload-pack) to a narrow one (git over SSH) we currently read limitless data into memory. Instead, throttle reads until writes catch up. This is now possible because of the previous changes in this sequence.

Test Plan:

• Ran git clone and git push on the entire Wine repository.
• Observed CPU and memory usage.
• Memory usage was constant and low, CPU usage was high only during I/O (which is expected, since we have to actually do work, although thre might be room to further reduce this).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4241, T4206

Differential Revision: https://secure.phabricator.com/D7776