Compile filter.c as C++

The contents of the existing filter.cc was copied into filter.c that
subsequently was renamed to filter.cc.

The way the session is called as the last filter in the filter chain
is really dubious and ought to be rearranged so that the blind casting
of a session to a filter and back is not needed.

server/core/maxscale/filter.h

@@ -44,6 +44,8 @@ MXS_DOWNSTREAM *filter_apply(MXS_FILTER_DEF *filter_def, MXS_SESSION *session, M
 void filter_free(MXS_FILTER_DEF *filter_def);
 bool filter_load(MXS_FILTER_DEF *filter_def);
 int filter_standard_parameter(const char *name);
-MXS_UPSTREAM *filter_upstream(MXS_FILTER_DEF *filter_def, void *fsession, MXS_UPSTREAM *upstream);
+MXS_UPSTREAM *filter_upstream(MXS_FILTER_DEF *filter_def,
+                              MXS_FILTER_SESSION *fsession,
+                              MXS_UPSTREAM *upstream);
 
 MXS_END_DECLS

server/core/maxscale/session.h

@@ -41,7 +41,6 @@ typedef enum
 } SESSIONLISTFILTER;
 
 int session_isvalid(MXS_SESSION *);
-int session_reply(void *inst, void *session, GWBUF *data);
 char *session_state(mxs_session_state_t);
 bool session_link_dcb(MXS_SESSION *, struct dcb *);