MEDIUM: checks: Associate a session to each tcp-check healthcheck

Create a session for each healthcheck relying on a tcp-check ruleset. When such
check is started, a session is allocated, which will be freed when the check
finishes. A dummy static frontend is used to create these sessions. This will be
useful to support variables and sample expression. This will also be used,
later, by HTTP healthchecks to rely on HTTP muxes.
diff --git a/include/types/checks.h b/include/types/checks.h
index 6826ac3..f92f0da 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -158,6 +158,7 @@
 };
 
 struct check {
+	struct session *sess;			/* Health check session. */
 	struct xprt_ops *xprt;			/* transport layer operations for health checks */
 	struct conn_stream *cs;			/* conn_stream state for health checks */
 	struct buffer bi, bo;			/* input and output buffers to send/recv check */