face: Implementing new "isOnDemand" flag

This flags replaces isPermanent flag in datagram faces, but with the
reversed logic (isPermanent = !isOnDemand)

Change-Id: I37ba604e5f105ca95a79a08b8cfc3d640df8b412
Refs: #1376
diff --git a/tests/face/tcp.cpp b/tests/face/tcp.cpp
index 38ee797..d908804 100644
--- a/tests/face/tcp.cpp
+++ b/tests/face/tcp.cpp
@@ -182,6 +182,9 @@
   BOOST_REQUIRE(static_cast<bool>(m_face1));
   BOOST_REQUIRE(static_cast<bool>(m_face2));
 
+  BOOST_CHECK(m_face1->isOnDemand());
+  BOOST_CHECK(!m_face2->isOnDemand());
+
   BOOST_CHECK_EQUAL(m_face2->getUri().toString(), "tcp4://127.0.0.1:20070");
   // face1 has an unknown URI, since the source port is automatically chosen by OS