MINOR: sink: add processed events counter in sft

Add a new struct member to sft structure named e_processed in order to
track the total number of events processed by sft applets.

sink_forward_oc_io_handler() and sink_forward_io_handler() now make use
of ring_dispatch_messages() optional value added in the previous commit
in order to increase the number of processed events.

include/haproxy/sink-t.h

@@ -40,6 +40,7 @@ struct sink_forward_target {
 	struct server *srv;    // used server
 	struct appctx *appctx; // appctx of current session
 	size_t ofs;            // ring buffer reader offset
+	size_t e_processed;    // processed events
 	struct sink *sink;     // the associated sink
 	struct sink_forward_target *next;
 	__decl_thread(HA_SPINLOCK_T lock); // lock to protect current struct