[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/src/proto_tcp.c b/src/proto_tcp.c
index a03c809..df6d034 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -686,7 +686,7 @@
 				buffer_abort(req);
 				buffer_abort(s->rep);
 				req->analysers = 0;
-				s->fe->failed_req++;
+				s->fe->counters.failed_req++;
 				if (!(s->flags & SN_ERR_MASK))
 					s->flags |= SN_ERR_PRXCOND;
 				if (!(s->flags & SN_FINST_MASK))