Register: kick out old session when number is taken

When a new session registers with an already-taken number, close the old connection. Prevents two devices from sharing the same number simultaneously.

internal/server/server.go

@@ -418,6 +418,9 @@ func marshalIdentityResponse(number string, certDER, caCertPEM []byte) []byte {
 func (s *Server) Register(sess call.CallSession, number string) {
 	s.sessionsMu.Lock()
 	defer s.sessionsMu.Unlock()
+	if old, ok := s.sessions[number]; ok && old != sess {
+		old.Conn().CloseWithError(0, "replaced by new session")
+	}
 	s.sessions[number] = sess.(*Session)
 }