safekeeper: use WAL_SEGMENT_SIZE for empty timeline state (#9734)

## Problem

`TimelinePersistentState::empty()`, used for tests and benchmarks, had a
hardcoded 16 MB WAL segment size. This caused confusion when attempting
to change the global segment size.

## Summary of changes

Inherit from `WAL_SEGMENT_SIZE` in `TimelinePersistentState::empty()`.

Author: erikgrinaker

safekeeper/src/state.rs

@@ -4,6 +4,7 @@
 use std::{cmp::max, ops::Deref};
 
 use anyhow::{bail, Result};
+use postgres_ffi::WAL_SEGMENT_SIZE;
 use safekeeper_api::models::TimelineTermBumpResponse;
 use serde::{Deserialize, Serialize};
 use utils::{
@@ -144,7 +145,7 @@ impl TimelinePersistentState {
             ServerInfo {
                 pg_version: 170000, /* Postgres server version (major * 10000) */
                 system_id: 0,       /* Postgres system identifier */
-                wal_seg_size: 16 * 1024 * 1024,
+                wal_seg_size: WAL_SEGMENT_SIZE as u32,
             },
             vec![],
             Lsn::INVALID,