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/modules/routing/debugcli/debugcmd.cc

@@ -63,6 +63,7 @@
 #include "../../../core/internal/monitor.h"
 #include "../../../core/internal/poll.h"
 #include "../../../core/internal/session.h"
+#include "../../../core/internal/filter.hh"
 
 #define MAXARGS 14