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

pkg/sfu/buffer/buffer.go

@@ -559,6 +559,9 @@ func (b *Buffer) SetSenderReportData(rtpTime uint32, ntpTime uint64) {
 }
 
 func (b *Buffer) SetLastFractionLostReport(lost uint8) {
+	b.Lock()
+	defer b.Unlock()
+
 	b.lastFractionLostToReport = lost
 }