[gold] Limit the number of go-routines in mem-diff-server
[gold] Limit the number of go-routines in mem-diff-server

Bug: skia:
Change-Id: Ia0e1b756249a9bb46bd4c0bcb4cd2fe478762b64
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/201883
Commit-Queue: Stephan Altmueller <43343ec95731c0e8daa7359daf7f2f3935582b24@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

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