Add sessions_alarm

Introduce new feature: sessions_alarm.

Added new account setting: max_connections. On exceeding this number,
user will get an alert. If number is less than 1, no alert will happen.

Tests altered to fit new feature.

src/xmpp/connection.c

@@ -776,6 +776,12 @@ connection_get_available_resources(void)
     return g_hash_table_get_values(conn.available_resources);
 }
 
+int
+connection_count_available_resources(void)
+{
+    return g_hash_table_size(conn.available_resources);
+}
+
 void
 connection_add_available_resource(Resource* resource)
 {