Avoid eprintln() in pageserver and walkeeper.

Use log::error!() instead. I spotted a few of these "connection error"
lines in the logs, without timestamps and the other stuff we print for
all other log messages.

pageserver/src/walreceiver.rs

@@ -160,7 +160,7 @@ fn walreceiver_main(
     // This is from tokio-postgres docs, but it is a bit weird in our case because we extensively use block_on
     runtime.spawn(async move {
         if let Err(e) = connection.await {
-            eprintln!("connection error: {}", e);
+            error!("connection error: {}", e);
         }
     });
 

walkeeper/src/callmemaybe.rs

@@ -33,7 +33,7 @@ async fn request_callback(
 
     tokio::spawn(async move {
         if let Err(e) = connection.await {
-            eprintln!("connection error: {}", e);
+            error!("connection error: {}", e);
         }
     });