admission: squash datarace in (*StoreWorkqueue).gcSequencers()

We see this (benign) data race in under --stress --race. Squash it by
targeted use of atomics.

  Read at 0x00c0039c06a8 by goroutine 31161:
    github.com/cockroachdb/cockroach/pkg/util/admission.(*StoreWorkQueue).gcSequencers()
        github.com/cockroachdb/cockroach/pkg/util/admission/work_queue.go:2176 +0x196
    github.com/cockroachdb/cockroach/pkg/util/admission.makeStoreWorkQueue.func1()
        github.com/cockroachdb/cockroach/pkg/util/admission/work_queue.go:2162 +0x5b

  Previous write at 0x00c0039c06a8 by goroutine 31105:
    github.com/cockroachdb/cockroach/pkg/util/admission.(*sequencer).sequence()
        github.com/cockroachdb/cockroach/pkg/util/admission/sequencer.go:61 +0x190
    github.com/cockroachdb/cockroach/pkg/util/admission.(*StoreWorkQueue).sequenceReplicatedWork()
        github.com/cockroachdb/cockroach/pkg/util/admission/work_queue.go:2193 +0x145
    github.com/cockroachdb/cockroach/pkg/util/admission.(*StoreWorkQueue).Admit()

Release note: None

pkg/util/admission/sequencer.go

@@ -10,6 +10,8 @@
 
 package admission
 
+import "sync/atomic"
+
 // sequencer issues monotonic sequencing timestamps derived from observed
 // CreateTimes. This is a purpose-built data structure for replication admission
 // control where we want to assign each AC-queued work below-raft a "sequence
@@ -58,6 +60,8 @@ func (s *sequencer) sequence(createTime int64) int64 {
 	if createTime <= s.maxCreateTime {
 		createTime = s.maxCreateTime + 1
 	}
-	s.maxCreateTime = createTime
+	// This counter is read concurrently in gcSequencers. We use an atomic store
+	// here and an atomic load there, to avoid tripping up the race detector.
+	atomic.StoreInt64(&s.maxCreateTime, createTime)
 	return createTime
 }

pkg/util/admission/work_queue.go

@@ -17,6 +17,7 @@ import (
 	"math"
 	"sort"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/cockroachdb/cockroach/pkg/base"
@@ -2173,7 +2174,7 @@ func (q *StoreWorkQueue) gcSequencers() {
 	defer q.sequencersMu.Unlock()
 
 	for rangeID, seq := range q.sequencersMu.s {
-		maxCreateTime := timeutil.FromUnixNanos(seq.maxCreateTime)
+		maxCreateTime := timeutil.FromUnixNanos(atomic.LoadInt64(&seq.maxCreateTime))
 		if q.timeSource.Now().Sub(maxCreateTime) > rangeSequencerGCThreshold.Get(&q.settings.SV) {
 			delete(q.sequencersMu.s, rangeID)
 		}