Various cleanups from anguslees@

[gyuho]

More idiomatic Rust :)

Split channel sender and receiver into two clear owners.

Replace sync crossbeam with async tokio channels, so we don't hold a
synchronous lock over async await.

Simplify applier start/stop lifecycle with RAII pattern (could do more here).

Pass owned echo::Manager to apply_async rather than Arc reference.

Use ? operator and early-return to refactor some deeply nested code,
particularly in server::handle_request.

Notifier: Replace async RwLock<> with simpler sync Arc<Mutex<>>
because these locks don't need to be held over async await.

echo::Manager: Use ownership (&mut) to assert mutual exclusion
rather than a lock.

echo::Request: Treat omitted message as equivalent to
message == "", to better match golang omitempty semantics.