feat(page_service): timeout-based batching of requests (#9321)

## Problem

We don't take advantage of queue depth generated by the compute
on the pageserver. We can process getpage requests more efficiently
by batching them. 

## Summary of changes

Batch up incoming getpage requests that arrive within a configurable
time window (`server_side_batch_timeout`).
Then process the entire batch via one `get_vectored` timeline operation.
By default, no merging takes place.

## Testing

* **Functional**: https://github.com/neondatabase/neon/pull/9792
* **Performance**: will be done in staging/pre-prod

# Refs

* https://github.com/neondatabase/neon/issues/9377
* https://github.com/neondatabase/neon/issues/9376

Co-authored-by: Christian Schwarz <christian@neon.tech>

libs/postgres_backend/src/lib.rs

@@ -716,6 +716,9 @@ impl<IO: AsyncRead + AsyncWrite + Unpin> PostgresBackend<IO> {
         Ok(())
     }
 
+    // Proto looks like this:
+    // FeMessage::Query("pagestream_v2{FeMessage::CopyData(PagesetreamFeMessage::GetPage(..))}")
+
     async fn process_message(
         &mut self,
         handler: &mut impl Handler<IO>,