common/gossip_store: optimize case where entries are filtered out.

@whitslack complained of large CPU usage by connectd at startup;
I ran perf record on connectd on my machine (which sees a little spike, only)
and I see the cost of reading and discarding the entries:

```
-   95.52%     5.24%  lightning_conne  lightning_connectd  [.] gossip_store_next
   - 90.28% gossip_store_next
      + 40.27% tal_alloc_arr_
      + 22.78% tal_free
      + 11.74% crc32c
      + 9.32% fromwire_peektype
      + 4.10% __libc_pread64 (inlined)
        1.70% be32_to_cpu
```

Much of this is caused by the search for our own gossip: keeping this separately
would be even better, but this fix is minimal.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: connectd: reduce initial CPU load when connecting to peers.

common/gossip_store.c

@@ -77,8 +77,16 @@ u8 *gossip_store_next(const tal_t *ctx,
 			continue;
 		}
 
-		checksum = be32_to_cpu(hdr.crc);
+		/* Skip any timestamp filtered */
 		timestamp = be32_to_cpu(hdr.timestamp);
+		if (!push &&
+		    !timestamp_filter(timestamp_min, timestamp_max,
+				      timestamp)) {
+			*off += r + msglen;
+			continue;
+		}
+
+		checksum = be32_to_cpu(hdr.crc);
 		msg = tal_arr(ctx, u8, msglen);
 		r = pread(*gossip_store_fd, msg, msglen, *off + r);
 		if (r != msglen)
@@ -105,10 +113,6 @@ u8 *gossip_store_next(const tal_t *ctx,
 			   && type != WIRE_CHANNEL_UPDATE
 			   && type != WIRE_NODE_ANNOUNCEMENT) {
 			msg = tal_free(msg);
-		} else if (!push &&
-			 !timestamp_filter(timestamp_min, timestamp_max,
-					   timestamp)) {
-			msg = tal_free(msg);
 		} else if (!push && push_only) {
 			msg = tal_free(msg);
 		}

gossipd/gossip_store.c

@@ -277,7 +277,7 @@ static u32 gossip_store_compact_offline(struct routing_state *rstate)
 	oldlen = lseek(old_fd, SEEK_END, 0);
 	newlen = lseek(new_fd, SEEK_END, 0);
 	append_msg(old_fd, towire_gossip_store_ended(tmpctx, newlen),
-		   0, false, &oldlen);
+		   0, true, &oldlen);
 	close(old_fd);
 	status_debug("gossip_store_compact_offline: %zu deleted, %zu copied",
 		     deleted, count);
@@ -565,7 +565,7 @@ bool gossip_store_compact(struct gossip_store *gs)
 
 	/* Write end marker now new one is ready */
 	append_msg(gs->fd, towire_gossip_store_ended(tmpctx, len),
-		   0, false, &gs->len);
+		   0, true, &gs->len);
 
 	gs->count = count;
 	gs->deleted = 0;