Fix a couple of more races (#717)

* Use grants clone

* Fix a couple of more races

Use a shadow copy of down tracks in DownTrackSpreader.
Read always uses the shadow.
On Add/Delete of down track, make a new copy.
Copying is done only on add/delete.
If somebody is holding reference to a shadow, it will be in tact as Add/Delete create a new slice.

With this, not seeing any more races in test. So, enabling CI tests with `-race`.

Also fixing another race reported in #603

There are a couple of more races in that bug report that needs to be
chased down.

* Use env suggested in https://lifesaver.codes/answer/runtime-race-detector-sigabrt-or-sigsegv-on-macos-monterey-49138

* staticcheck, did not fail locally, but reported by CI

* use API to get down tracks

.github/workflows/buildtest.yaml

@@ -57,7 +57,7 @@ jobs:
       - name: Test
         run: |
           set -euo pipefail
-          go test -json -v ./... 2>&1 | tee /tmp/gotest.log | gotestfmt
+          MallocNanoZone=0 go test -race -json -v ./... 2>&1 | tee /tmp/gotest.log | gotestfmt
 
       # Upload the original go test log as an artifact for later review.
       - name: Upload test log