re-use rand.New() do not repeat allocate. (#13448)

also simplify readerLocks to be just like
writeLocks, DRWMutex() is never shared
and there are order guarantees that need
for such a thing to work for RLock's

internal/dsync/drwmutex_test.go

@@ -306,6 +306,9 @@ func TestRUnlockPanic2(t *testing.T) {
 
 // Borrowed from rwmutex_test.go
 func benchmarkRWMutex(b *testing.B, localWork, writeRatio int) {
+	b.ResetTimer()
+	b.ReportAllocs()
+
 	rwm := NewDRWMutex(ds, "test")
 	b.RunParallel(func(pb *testing.PB) {
 		foo := 0