startTimeMargin and StartTime is used only by Prometheus remote read, Thanos does not use it.
Fixed following race:
```
=== RUN TestMultiTSDB/run_on_existing_storage
==================
WARNING: DATA RACE
Read at 0x00c00073ae80 by goroutine 69:
github.com/prometheus/prometheus/tsdb.validateOpts()
/home/bwplotka/Repos/thanosgopath/pkg/mod/github.com/prometheus/[email protected]/tsdb/db.go:510 +0x55
github.com/prometheus/prometheus/tsdb.Open()
/home/bwplotka/Repos/thanosgopath/pkg/mod/github.com/prometheus/[email protected]/tsdb/db.go:502 +0x61
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).getOrLoadTenant.func1()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:268 +0x56b
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).getOrLoadTenant()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:302 +0x4ef
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).Open.func1()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:142 +0x66
golang.org/x/sync/errgroup.(*Group).Go.func1()
/home/bwplotka/Repos/thanosgopath/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:57 +0x85
Previous write at 0x00c00073ae80 by goroutine 57:
github.com/prometheus/prometheus/tsdb.validateOpts()
/home/bwplotka/Repos/thanosgopath/pkg/mod/github.com/prometheus/[email protected]/tsdb/db.go:511 +0x1f2
github.com/prometheus/prometheus/tsdb.Open()
/home/bwplotka/Repos/thanosgopath/pkg/mod/github.com/prometheus/[email protected]/tsdb/db.go:502 +0x61
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).getOrLoadTenant.func1()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:268 +0x56b
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).getOrLoadTenant()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:302 +0x4ef
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).Open.func1()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:142 +0x66
golang.org/x/sync/errgroup.(*Group).Go.func1()
/home/bwplotka/Repos/thanosgopath/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:57 +0x85
Goroutine 69 (running) created at:
golang.org/x/sync/errgroup.(*Group).Go()
/home/bwplotka/Repos/thanosgopath/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:54 +0x73
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).Open()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:141 +0x2af
github.com/thanos-io/thanos/pkg/receive.TestMultiTSDB.func3()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb_test.go:118 +0x6d3
testing.tRunner()
/home/bwplotka/.gvm/gos/go1.14.2/src/testing/testing.go:991 +0x1eb
Goroutine 57 (running) created at:
golang.org/x/sync/errgroup.(*Group).Go()
/home/bwplotka/Repos/thanosgopath/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:54 +0x73
github.com/thanos-io/thanos/pkg/receive.(*MultiTSDB).Open()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb.go:141 +0x2af
github.com/thanos-io/thanos/pkg/receive.TestMultiTSDB.func3()
/home/bwplotka/Repos/thanos/pkg/receive/multitsdb_test.go:118 +0x6d3
testing.tRunner()
/home/bwplotka/.gvm/gos/go1.14.2/src/testing/testing.go:991 +0x1eb
==================
```
Signed-off-by: Bartlomiej Plotka <[email protected]>