Shrink thread pools used for WAL receivers and background tasks.

I noticed that the pageserver has a very large virtual memory size,
several GB, even though it doesn't actually use that much
memory. That's not much of a problem normally, but I hit it because I
wanted to run tests with a limited virtual memory size, by calling
setrlimit(RLIMIT_AS), but the highest limit you can set is 2 GB. I was
not able to start pageserver with a limit of 2 GB.

On Linux, each thread allocates 32 MB of virtual memory. I read this
on some random forum on the Internet, but unfortunately could not find
the source again now. Empirically, reducing the number of threads clearly
helps to bring down the virtual memory size.

Aside from the virtual memory usage, it seems excessive to launch 40
threads in both of those thread pools. The tokio default is to have as
many worker threads as there are CPU cores in the system. That seems
like a fine heuristic for us, too, so remove the explicit setting of
the pool size and rely on the default. Note that the GC and compaction
tasks are actually run with tokio spawn_blocking, so the threads that
are actually doing the work, and possibly waiting on I/O, are not
consuming threads from the thread pool. The WAL receiver work is done
in the tokio worker threads, but the WAL receivers are more CPU bound
so that seems OK.

Also remove the explicit maxinum on blocking tasks. I'm not sure what
the right value for that would be, or whether the value we set (100)
would be better than the tokio default (512). Since the value was
arbitrary, let's just rely on the tokio default for that, too.

pageserver/src/tenant_tasks.rs

@@ -119,8 +119,6 @@ pub fn start_compaction_loop(tenantid: ZTenantId) -> anyhow::Result<()> {
 pub fn init_tenant_task_pool() -> anyhow::Result<()> {
     let runtime = tokio::runtime::Builder::new_multi_thread()
         .thread_name("tenant-task-worker")
-        .worker_threads(40) // Way more than necessary
-        .max_blocking_threads(100) // Way more than necessary
         .enable_all()
         .build()?;
 

pageserver/src/walreceiver.rs

@@ -91,7 +91,6 @@ pub fn init_wal_receiver_main_thread(
 
     let runtime = tokio::runtime::Builder::new_multi_thread()
         .thread_name("wal-receiver-runtime-thread")
-        .worker_threads(40)
         .enable_all()
         .on_thread_start(|| IS_WAL_RECEIVER.with(|c| c.set(true)))
         .build()