query/endpointset: fix some races (#5972)

Fix races around reading `e.endpoints`. Example race:

```
==================
WARNING: DATA RACE
Write at 0x00c00006a668 by main goroutine:
  github.com/thanos-io/thanos/pkg/query.(*EndpointSet).Close()
      /home/giedrius/dev/thanos/pkg/query/endpointset.go:582 +0x164
  main.runQuery.func3()
      /home/giedrius/dev/thanos/cmd/thanos/query.go:534 +0x44
  github.com/oklog/run.(*Group).Run()
      /home/giedrius/go/pkg/mod/github.com/oklog/run@v1.1.0/group.go:47 +0x1ce
  main.main()
      /home/giedrius/dev/thanos/cmd/thanos/main.go:159 +0x2669

Previous read at 0x00c00006a668 by goroutine 276:
  github.com/thanos-io/thanos/pkg/query.(*EndpointSet).Update()
      /home/giedrius/dev/thanos/pkg/query/endpointset.go:396 +0xb9d
  main.runQuery.func2.1()
      /home/giedrius/dev/thanos/cmd/thanos/query.go:529 +0x4c
  github.com/thanos-io/thanos/pkg/runutil.Repeat()
      /home/giedrius/dev/thanos/pkg/runutil/runutil.go:74 +0xd8
  main.runQuery.func2()
      /home/giedrius/dev/thanos/cmd/thanos/query.go:528 +0x99
  github.com/oklog/run.(*Group).Run.func1()
      /home/giedrius/go/pkg/mod/github.com/oklog/run@v1.1.0/group.go:38 +0x41
  github.com/oklog/run.(*Group).Run.func2()
      /home/giedrius/go/pkg/mod/github.com/oklog/run@v1.1.0/group.go:39 +0x58

Goroutine 276 (running) created at:
  github.com/oklog/run.(*Group).Run()
      /home/giedrius/go/pkg/mod/github.com/oklog/run@v1.1.0/group.go:37 +0x349
  main.main()
      /home/giedrius/dev/thanos/cmd/thanos/main.go:159 +0x2669
==================
```

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

pkg/query/endpointset.go

@@ -393,6 +393,7 @@ func (e *EndpointSet) Update(ctx context.Context) {
 	wg.Wait()
 
 	timedOutRefs := e.getTimedOutRefs()
+	e.endpointsMtx.RLock()
 	for addr, er := range e.endpoints {
 		_, isNew := newRefs[addr]
 		_, isExisting := existingRefs[addr]
@@ -401,6 +402,7 @@ func (e *EndpointSet) Update(ctx context.Context) {
 			staleRefs[addr] = er
 		}
 	}
+	e.endpointsMtx.RUnlock()
 
 	e.endpointsMtx.Lock()
 	defer e.endpointsMtx.Unlock()
@@ -450,6 +452,8 @@ func (e *EndpointSet) updateEndpoint(ctx context.Context, spec *GRPCEndpointSpec
 // successful health check is older than the unhealthyEndpointTimeout.
 // Strict endpoints are never considered as timed out.
 func (e *EndpointSet) getTimedOutRefs() map[string]*endpointRef {
+	e.endpointsMtx.RLock()
+	defer e.endpointsMtx.RUnlock()
 	result := make(map[string]*endpointRef)
 
 	endpoints := e.endpoints