[CLEANUP] Move counters to dedicated structures

Move counters from "struct proxy" and "struct server"
to "struct pxcounters" and "struct svcounters".

This patch should make no functional change.
diff --git a/include/proto/server.h b/include/proto/server.h
index 7479c2e..43d9216 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -38,7 +38,7 @@
 /* increase the number of cumulated connections on the designated server */
 static void inline srv_inc_sess_ctr(struct server *s)
 {
-	s->cum_sess++;
+	s->counters.cum_sess++;
 	update_freq_ctr(&s->sess_per_sec, 1);
 	if (s->sess_per_sec.curr_ctr > s->sps_max)
 		s->sps_max = s->sess_per_sec.curr_ctr;