And add a test to cover that.
The data race would look roughly like:
```
WARNING: DATA RACE
Write at 0x00c000024e18 by goroutine 8:
github.com/RoaringBitmap/roaring.(*roaringArray).markAllAsNeedingCopyOnWrite()
/Users/mr/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.4.16/roaringarray.go:881 +0x6b
github.com/RoaringBitmap/roaring.(*roaringArray).clone()
/Users/mr/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.4.16/roaringarray.go:266 +0x808
github.com/RoaringBitmap/roaring.(*Bitmap).Clone()
/Users/mr/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.4.16/roaring.go:385 +0x58
github.com/influxdata/platform/tsdb.(*SeriesIDSet).CloneNoLock()
/Users/mr/go/src/github.com/influxdata/platform/tsdb/series_set.go:229 +0x73
github.com/influxdata/platform/tsdb.(*SeriesIDSet).Clone()
Previous write at 0x00c000024e18 by goroutine 7:
github.com/RoaringBitmap/roaring.(*roaringArray).markAllAsNeedingCopyOnWrite()
/Users/mr/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.4.16/roaringarray.go:881 +0x6b
github.com/RoaringBitmap/roaring.(*roaringArray).clone()
/Users/mr/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.4.16/roaringarray.go:266 +0x808
github.com/RoaringBitmap/roaring.(*Bitmap).Clone()
/Users/mr/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.4.16/roaring.go:385 +0x58
github.com/influxdata/platform/tsdb.(*SeriesIDSet).CloneNoLock()
/Users/mr/go/src/github.com/influxdata/platform/tsdb/series_set.go:229 +0x73
github.com/influxdata/platform/tsdb.(*SeriesIDSet).Clone()
/Users/mr/go/src/github.com/influxdata/platform/tsdb/series_set.go:223 +0x7b
```