RPC: Mark storage methods as `blocking` (#11459)

* client/api: Make `storage_keys` blocking

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client/api: Ensure `state_*` RPC methods are blocking

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client/rpc: Ensure `childstate_*` RPC methods are blocking

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client/rpc: `ChainApi` make RPC methods sync

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Remove unused async-traits

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* client/rpc-api: Make chain RPC methods blocking

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update client/rpc/src/state/state_full.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Add `blocking` to `state_getKeysPaged` RPC call

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Fix build and warning

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Remove `async_trait` tidyup

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>