syncs: add Mutex/RWMutex alias/wrappers for future mutex debugging

Updates #17852

Change-Id: I477340fb8e40686870e981ade11cd61597c34a20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlbase/conn.go

@@ -18,6 +18,7 @@ import (
 
 	"golang.org/x/crypto/blake2s"
 	chp "golang.org/x/crypto/chacha20poly1305"
+	"tailscale.com/syncs"
 	"tailscale.com/types/key"
 )
 
@@ -48,7 +49,7 @@ type Conn struct {
 
 // rxState is all the Conn state that Read uses.
 type rxState struct {
-	sync.Mutex
+	syncs.Mutex
 	cipher    cipher.AEAD
 	nonce     nonce
 	buf       *maxMsgBuffer   // or nil when reads exhausted