pageserver: option to run with just one tokio runtime (#7331)

This PR is an off-by-default revision v2 of the (since-reverted) PR
#6555 / commit `3220f830b7fbb785d6db8a93775f46314f10a99b`.

See that PR for details on why running with a single runtime is
desirable and why we should be ready.

We reverted #6555 because it showed regressions in prodlike cloudbench,
see the revert commit message `ad072de4209193fd21314cf7f03f14df4fa55eb1`
for more context.

This PR makes it an opt-in choice via an env var.

The default is to use the 4 separate runtimes that we have today, there
shouldn't be any performance change.

I tested manually that the env var & added metric works.

```
# undefined env var => no change to before this PR, uses 4 runtimes
./target/debug/neon_local start
# defining the env var enables one-runtime mode, value defines that one runtime's configuration
NEON_PAGESERVER_USE_ONE_RUNTIME=current_thread ./target/debug/neon_local start
NEON_PAGESERVER_USE_ONE_RUNTIME=multi_thread:1 ./target/debug/neon_local start
NEON_PAGESERVER_USE_ONE_RUNTIME=multi_thread:2 ./target/debug/neon_local start
NEON_PAGESERVER_USE_ONE_RUNTIME=multi_thread:default ./target/debug/neon_local start

```

I want to use this change to do more manualy testing and potentially
testing in staging.

Future Work
-----------

Testing / deployment ergonomics would be better if this were a variable
in `pageserver.toml`.
It can be done, but, I don't need it right now, so let's stick with the
env var.

libs/utils/src/env.rs

@@ -0,0 +1,21 @@
+//! Wrapper around `std::env::var` for parsing environment variables.
+
+use std::{fmt::Display, str::FromStr};
+
+pub fn var<V, E>(varname: &str) -> Option<V>
+where
+    V: FromStr<Err = E>,
+    E: Display,
+{
+    match std::env::var(varname) {
+        Ok(s) => Some(
+            s.parse()
+                .map_err(|e| format!("failed to parse env var {varname}: {e:#}"))
+                .unwrap(),
+        ),
+        Err(std::env::VarError::NotPresent) => None,
+        Err(std::env::VarError::NotUnicode(_)) => {
+            panic!("env var {varname} is not unicode")
+        }
+    }
+}

libs/utils/src/lib.rs

@@ -89,6 +89,8 @@ pub mod yielding_loop;
 
 pub mod zstd;
 
+pub mod env;
+
 /// This is a shortcut to embed git sha into binaries and avoid copying the same build script to all packages
 ///
 /// we have several cases: