[compute_ctl] Empty computes and /configure API (#3963)

This commit adds an option to start compute without spec and then pass
it a valid spec via `POST /configure` API endpoint. This is a main
prerequisite for maintaining the pool of compute nodes in the
control-plane.

For example:

1. Start compute with
   ```shell
   cargo run --bin compute_ctl -- -i no-compute \
    -p http://localhost:9095 \
    -D compute_pgdata \
    -C "postgresql://cloud_admin@127.0.0.1:5434/postgres" \
    -b ./pg_install/v15/bin/postgres
   ```

2. Configure it with
   ```shell
   curl -d "{\"spec\": $(cat ./compute-spec.json)}" http://localhost:3080/configure
   ```

Internally, it's implemented using a `Condvar` + `Mutex`. Compute spec
is moved under Mutex, as it's now could be updated in the http handler.
Also `RwLock` was replaced with `Mutex` because the latter works well
with `Condvar`.

First part of the neondatabase/cloud#4433

compute_tools/src/monitor.rs

@@ -46,7 +46,7 @@ fn watch_compute_activity(compute: &ComputeNode) {
                             AND usename != 'cloud_admin';", // XXX: find a better way to filter other monitors?
                         &[],
                     );
-                let mut last_active = compute.state.read().unwrap().last_active;
+                let mut last_active = compute.state.lock().unwrap().last_active;
 
                 if let Ok(backs) = backends {
                     let mut idle_backs: Vec<DateTime<Utc>> = vec![];
@@ -87,7 +87,7 @@ fn watch_compute_activity(compute: &ComputeNode) {
                 }
 
                 // Update the last activity in the shared state if we got a more recent one.
-                let mut state = compute.state.write().unwrap();
+                let mut state = compute.state.lock().unwrap();
                 if last_active > state.last_active {
                     state.last_active = last_active;
                     debug!("set the last compute activity time to: {}", last_active);