`derive` should require methods to be `async`

[tomusdrw]

Currently the jsonrpc-derive crate can only work with either Result or BoxFuture return types (see #565). Unfortunatelly traits don't support async methods yet, but there are polyfills like async-trait.

Long term, since derive is more opinionated than core, I think we should get rid of synchronous methods and require the jsonrpc-derive traits to have all methods async. This will simplify code handling (see WrapFuture) and would allow returning custom futures without Box::pining them.