Cache initdb output to speed up tenant creation in tests

initdb takes about 1 s. Our tests create and destroy a lot of tenants,
so that adds up. Cache the initdb result to speed it up.

This is currently only enabled in tests. Out of caution, mostly. But
also because when you reuse the initdb result, all the postgres
clusters end up having the same system_identifier, which is supposed
to be unique. It's not necessary for it to be unique for correctness,
nothing critical relies on it and you can easily end up with duplicate
system_identifiers in standalone PostgreSQL too, if you e.g. create a
backup and restore it on a different system. But it is used in various
checks to reduce the chance that you e.g. accidentally apply WAL
belonging to a different cluster.

Because this is aimed at tests, there are a few things that might be
surprising:

- The initdb cache directory is configurable, and can be outside the
  pageserver's repo directory. This allows reuse across different
  pageservers running on the same host. In production use, that'd be
  pointless, but our tests create a lot of pageservers.

- The cache is not automatically purged at start / shutdown. For
  production use, we'd probably want that, so that we'd pick up any
  changes in what an empty cluster looks like after a Postgres minor
  version upgrade, for example. But again tests create and destroy a
  lot of pageservers, so it's important to retain the cache.

- The locking on the cache directory relies purely on filesystem
  operations and atomic rename(). Using e.g. a rust Mutex() would be
  more straightforward, but that's not enough because the cache needs
  to be shared between different pageservers running on the same
  system.

libs/pageserver_api/src/config.rs

@@ -64,6 +64,7 @@ pub struct ConfigToml {
     #[serde(with = "humantime_serde")]
     pub wal_redo_timeout: Duration,
     pub superuser: String,
+    pub initdb_cache_dir: Option<Utf8PathBuf>,
     pub page_cache_size: usize,
     pub max_file_descriptors: usize,
     pub pg_distrib_dir: Option<Utf8PathBuf>,
@@ -358,6 +359,7 @@ impl Default for ConfigToml {
             wal_redo_timeout: (humantime::parse_duration(DEFAULT_WAL_REDO_TIMEOUT)
                 .expect("cannot parse default wal redo timeout")),
             superuser: (DEFAULT_SUPERUSER.to_string()),
+            initdb_cache_dir: None,
             page_cache_size: (DEFAULT_PAGE_CACHE_SIZE),
             max_file_descriptors: (DEFAULT_MAX_FILE_DESCRIPTORS),
             pg_distrib_dir: None, // Utf8PathBuf::from("./pg_install"), // TODO: formely, this was std::env::current_dir()