Switch from `block_in_place` to `spawn_blocking` for async operations

[neysofu]

We currently use tokio::task::block_in_place for async operations within Rockbound, but we'd probably like to switch to spawn_blocking. spawn_blocking has some performance benefits and, most importantly, doesn't abort in case of panics but instead recovers from them.

[neysofu]

When this is solved, I believe we can remove all (flavor = "multi_thread") attributes from Rockbound as well as sovereign-sdk.