MINOR: snapshot: restart on the event ID and not the stream ID

The snapshots have the ability to restart a partial dump and they use
the stream ID as the restart point. Since it's purely HTTP, let's use
the event ID instead.
diff --git a/src/proto_http.c b/src/proto_http.c
index cf759fb..b6d429b 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -12784,10 +12784,10 @@
 				return 0;
 			}
 			appctx->ctx.errors.ptr = 0;
-			appctx->ctx.errors.sid = es->sid;
+			appctx->ctx.errors.ev_id = es->ev_id;
 		}
 
-		if (appctx->ctx.errors.sid != es->sid) {
+		if (appctx->ctx.errors.ev_id != es->ev_id) {
 			/* the snapshot changed while we were dumping it */
 			chunk_appendf(&trash,
 				     "  WARNING! update detected on this snapshot, dump interrupted. Please re-check!\n");