safekeeper: downgrade stream from future WAL log (#11909)

## Problem

1. Safekeeper selection on the pageserver side isn't very dynamic. Once
you connect to one safekeeper, you'll use that one for as long as the
safekeeper keeps the connection alive. In principle, we could be more
eager, since the wal receiver connection can be cancelled but we don't
do that. We wait until the "session" is done and then we pick a new SK.
2. Picking a new SK is quite conservative. We will switch if: 
a. We haven't received anything from the SK within the last 10 seconds
(wal_connect_timeout) or
b. The candidate SK is 1GiB ahead or
c. The candidate SK is in the same AZ as the PS or d. There's a
candidate that is ahead and we've not had any WAL within the last 10
seconds (lagging_wal_timeout)

Hence, we can end up with pageservers that are requesting WAL which
their safekeeper hasn't seen yet.

## Summary of changes

Downgrade warning log to info.

safekeeper/src/send_wal.rs

@@ -513,7 +513,7 @@ impl SafekeeperPostgresHandler {
         let end_pos = end_watch.get();
 
         if end_pos < start_pos {
-            warn!(
+            info!(
                 "requested start_pos {} is ahead of available WAL end_pos {}",
                 start_pos, end_pos
             );