vsl: New Timestamp:Process for backend tasks

Similarly to Timestamp:Fetch, it is otherwise not possible to distinguish
between time spent in VCL and time spent strictly fetching the response
body.

bin/varnishd/cache/cache_fetch.c

@@ -518,6 +518,7 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 		return (F_STP_ERROR);
 	}
 
+	VSLb_ts_busyobj(bo, "Process", W_TIM_real(wrk));
 	assert(oc->boc->state <= BOS_REQ_DONE);
 	if (oc->boc->state != BOS_REQ_DONE) {
 		bo->req = NULL;

doc/sphinx/reference/vsl.rst

@@ -98,6 +98,9 @@ Bereq
 Beresp
 	Backend response headers received.
 
+Process
+	Processing finished, ready to fetch the response body.
+
 BerespBody
 	Backend response body received.