Instead of spawning OS threads manually, use tokio::spawn, which schedules tasks onto the Tokio runtime's async task pool.

[ametel01]

The spawned thread then creates a new tokio::runtime::Runtime, which is unnecessary because you're already inside a Tokio async runtime (via #[tokio::main]).

Changes should be applied to the following:

• fossil-headers-db/src/indexer/main.rs

  Line 72 in d9001f9

  let router_handle = thread::Builder::new()

• fossil-headers-db/src/indexer/main.rs

  Line 98 in d9001f9

  let quick_indexer_handle = thread::Builder::new()

• fossil-headers-db/src/indexer/main.rs

  Line 122 in d9001f9

  let batch_indexer_handle = thread::Builder::new()