MXS-1929: Make filters fully opaque

The filter implementation is now fully hidden. Also converted it to a C++
struct allocated with new and stored the filters in a global list instead
of embedding the list in the object itself.

server/core/config.cc

@@ -52,7 +52,7 @@
 
 #include "internal/config.h"
 #include "internal/event.hh"
-#include "internal/filter.h"
+#include "internal/filter.hh"
 #include "internal/modules.h"
 #include "internal/monitor.h"
 #include "internal/service.h"