Limit the number of goroutines spawned off by gs.go.
Limit the number of goroutines spawned off by gs.go.

This is to try to stay below the storage API qps limit.

BUG=skia:3163

Review URL: https://codereview.chromium.org/853653006

Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot