MINOR: session: add stick counters to the struct session

The stick counters in the session will be used for everything not related
to contents, hence the connections / concurrent sessions / etc. They will
be usable by "tcp-request connection" rules even without a stream. For now
they're just allocated and initialized.
diff --git a/src/peers.c b/src/peers.c
index df4ecf8..57954d7 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1127,6 +1127,7 @@
 	sess->fe = p;
 	sess->accept_date = date; /* user-visible date for logging */
 	sess->tv_accept = now;  /* corrected date for internal use */
+	memset(sess->stkctr, 0, sizeof(sess->stkctr));
 
 	if ((s = pool_alloc2(pool2_stream)) == NULL) { /* disable this proxy for a while */
 		Alert("out of memory in peer_session_create().\n");