Turn lite::types::Requester into an async trait

[romac]

Requester not being an async trait forces implementations to block on RPC queries, eg. by spawning a new executor, which is undesirable considering that we eventually want the whole codebase to play nice with async/.await, and not unnecessarily block the enclosing task.

This can be accomplished with async-trait crate, which turns async fns within traits and their implementations into plain functions returning Pin<Box<dyn Future<Output = T + ...>>>.

The obvious drawback of this approach is that these async Fns now allocate memory on the heap. Hopefully, the language will soon stabilize the feature, which will help us get rid of these undesirable heap allocations.