Other(refactor): Sort safe.Mutex types before locking to prevent deadlocks

This change implements safe.Mutex and safe.RWMutex, which wrap the
sync.Mutex and sync.RWMutex types and are assigned a globally unique
integer ID. The safe.Lock and safe.RLock methods sort the mutexes
by this integer ID before locking to ensure that locks for a given
set of mutexes are always performed in the same order, avoiding
deadlocks.

internal/bridge/smtp_backend.go

@@ -58,7 +58,7 @@ func (s *smtpSession) AuthPlain(username, password string) error {
 		}
 
 		return fmt.Errorf("invalid username or password")
-	}, &s.usersLock)
+	}, s.usersLock)
 }
 
 func (s *smtpSession) Reset() {
@@ -92,5 +92,5 @@ func (s *smtpSession) Data(r io.Reader) error {
 		}
 
 		return user.SendMail(s.authID, s.from, s.to, r)
-	}, &s.usersLock)
+	}, s.usersLock)
 }