Add parallelism to filediffstore. Spawn X number of goroutines. X is determined by the WorkerPoolNum parameter.

How I tested:
Created a directory in gs://chromium-skia-gm/concur-testdata/ with lots of images. Manually specified 44 digests in filediffstore_test.MassiveTestGet and filediffstore_test.MassiveTestAbsPath.

Before my change MassiveTestGet took ~20 seconds to run. After my change it takes ~12 seconds to run. That is a 40% improvement.
Before my change MassiveTestAbsPath took ~7 seconds to run. After my change it takes ~2 seconds to run. That is a 70% improvement.

Other changes in this CL:
* Broke up the single mutex into one for the diff dir and one for the digests dir.
* The diff mutex was not locked during writing, fixed this.
* Documented the algorithm used in AbsPathAsync.

BUG=skia:2930

Review URL: https://codereview.chromium.org/660453003