device: switch to regular mutex for keypair guard

name                              old time/op      new time/op      delta
TrieIPv4Peers100Addresses1000-32      83.5ns ± 0%      83.3ns ± 0%   ~     (p=1.000 n=1+1)
TrieIPv4Peers10Addresses10-32         33.6ns ± 0%      33.4ns ± 0%   ~     (p=1.000 n=1+1)
TrieIPv6Peers100Addresses1000-32      83.3ns ± 0%      83.2ns ± 0%   ~     (p=1.000 n=1+1)
TrieIPv6Peers10Addresses10-32         33.5ns ± 0%      33.2ns ± 0%   ~     (p=1.000 n=1+1)
Latency-32                             216µs ± 0%       216µs ± 0%   ~     (p=1.000 n=1+1)
Throughput-32                         2.31µs ± 0%      2.28µs ± 0%   ~     (p=1.000 n=1+1)
UAPIGet-32                            2.28µs ± 0%      2.13µs ± 0%   ~     (p=1.000 n=1+1)
WaitPool-32                           4.18µs ± 0%      4.14µs ± 0%   ~     (p=1.000 n=1+1)

name                              old packet-loss  new packet-loss  delta
Throughput-32                           0.00 ± 0%        0.01 ± 0%   ~     (p=1.000 n=1+1)

name                              old alloc/op     new alloc/op     delta
UAPIGet-32                              224B ± 0%        224B ± 0%   ~     (all equal)

name                              old allocs/op    new allocs/op    delta
UAPIGet-32                              17.0 ± 0%        17.0 ± 0%   ~     (all equal)

device/device.go

@@ -392,10 +392,8 @@ func (device *Device) SendKeepalivesToPeersWithCurrentKeypair() {
 
 	device.peers.RLock()
 	for _, peer := range device.peers.keyMap {
-		peer.keypairs.RLock()
-		sendKeepalive := peer.keypairs.current != nil && !peer.keypairs.current.created.Add(RejectAfterTime).Before(time.Now())
-		peer.keypairs.RUnlock()
-		if sendKeepalive {
+		current := peer.keypairs.Current()
+		if current.created.Add(RejectAfterTime).Before(time.Now()) {
 			peer.SendKeepalive()
 		}
 	}

device/keypair.go

@@ -33,15 +33,15 @@ type Keypair struct {
 }
 
 type Keypairs struct {
-	sync.RWMutex
+	sync.Mutex
 	current  *Keypair
 	previous *Keypair
 	next     *Keypair
 }
 
 func (kp *Keypairs) Current() *Keypair {
-	kp.RLock()
-	defer kp.RUnlock()
+	kp.Lock()
+	defer kp.Unlock()
 	return kp.current
 }