satellite/repair: protect concurrent access to statsCollector

It would appear that we have been making concurrent accesses to
statsCollector for a long, long time (we expect there to be multiple
calls to `Repair()` at the same time on the same instance of
`SegmentRepairer`, up to `config.MaxRepair`, and before this change
there was no sort of synchronization guarding accesses to the
`statsCollector.stats` map.

Refs: https://github.com/storj/storj/issues/6402
Change-Id: I5bcdd13c88913a8d66f6dd906c9037c588960cc9

satellite/repair/repairer/stats.go

@@ -5,6 +5,7 @@ package repairer
 
 import (
 	"fmt"
+	"sync"
 
 	"github.com/spacemonkeygo/monkit/v3"
 )
@@ -13,6 +14,7 @@ import (
 // seen by the repairer. These are chained into the monkit scope for
 // monitoring as they are initialized.
 type statsCollector struct {
+	lock  sync.Mutex
 	stats map[string]*stats
 }
 
@@ -23,6 +25,9 @@ func newStatsCollector() *statsCollector {
 }
 
 func (collector *statsCollector) getStatsByRS(rs string) *stats {
+	collector.lock.Lock()
+	defer collector.lock.Unlock()
+
 	stats, ok := collector.stats[rs]
 	if !ok {
 		stats = newStats(rs)