* 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
