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/pageserver_api/src/config.rs

@@ -109,6 +109,8 @@ pub struct ConfigToml {
     pub virtual_file_io_mode: Option<crate::models::virtual_file::IoMode>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub no_sync: Option<bool>,
+    #[serde(with = "humantime_serde")]
+    pub server_side_batch_timeout: Option<Duration>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
@@ -317,6 +319,8 @@ pub mod defaults {
     pub const DEFAULT_EPHEMERAL_BYTES_PER_MEMORY_KB: usize = 0;
 
     pub const DEFAULT_IO_BUFFER_ALIGNMENT: usize = 512;
+
+    pub const DEFAULT_SERVER_SIDE_BATCH_TIMEOUT: Option<&str> = None;
 }
 
 impl Default for ConfigToml {
@@ -397,6 +401,8 @@ impl Default for ConfigToml {
             ephemeral_bytes_per_memory_kb: (DEFAULT_EPHEMERAL_BYTES_PER_MEMORY_KB),
             l0_flush: None,
             virtual_file_io_mode: None,
+            server_side_batch_timeout: DEFAULT_SERVER_SIDE_BATCH_TIMEOUT
+                .map(|duration| humantime::parse_duration(duration).unwrap()),
             tenant_config: TenantConfigToml::default(),
             no_sync: None,
         }