Summary:
A long time ago, getdeps scheduled each build up to the number of hardware threads. For some builds, that was too heavy, so it got throttled back to only ever use half the hardware threads. This left parallelism on the table for CPU-bound projects that don't use much RAM per compilation.

This commit makes better use of the hardware with finer-grained logic that allows each manifest to specify a `job_weight_mib` estimate in MiB, and limit concurrency to `available_ram / job_weight`.

Reviewed By: ahornby

Differential Revision: D33754018

fbshipit-source-id: 785bed6c6cfe3c473244e0806a77cec1fc119e1f