Add very basic Stream Management (XEP-0198) support

This implementation only counts incomming stanzas and
sends ACKs on request.
While basic this already allows servers to write
messages to offline storage that were sent to,
but never received by a client.

src/XMPPConnection.h

@@ -158,6 +158,7 @@
 	BOOL needsSession;
 	BOOL encryptionRequired, encrypted;
 	BOOL supportsRosterVersioning;
+	BOOL supportsStreamManagement;
 	unsigned int lastID;
 /// \endcond
 }
@@ -197,6 +198,8 @@
 @property (readonly) BOOL encrypted;
 /// \brief Whether roster versioning is supported
 @property (readonly) BOOL supportsRosterVersioning;
+/// \brief Whether stream management is supported
+@property (readonly) BOOL supportsStreamManagement;
 #endif
 
 /**