Support `account_nextIndex` RPC. (#460)

* Use node-rpc extensions to support account_nextIndex.

* Remove todo.

* Update lock.

* Use new srml_system_rpc crate.

* Update to substrate=master

* Update lockfile.

* Update to polkadot-master.

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>