WIP XEP-0198: implement stream management

Stream management is enabled when server supports it. There is no option
to disable it.

strophe.h

@@ -75,6 +75,10 @@ extern "C" {
  *  Namespace definition for 'jabber:iq:register'.
  */
 #define XMPP_NS_REGISTER "jabber:iq:register"
+/** @def XMPP_NS_SM
+ *  Namespace definition for Stream Management.
+ */
+#define XMPP_NS_SM "urn:xmpp:sm:3"
 
 /* error defines */
 /** @def XMPP_EOK