session: add handler for genesis session (#3413)

* session: add handler for genesis session

* node: bump spec version

* aura: handle on_genesis_session

* srml: make sure we don't re-initialize genesis authorities

* session: fix mock

* node: remove genesis authorities from chain spec

* staking: fix mock

* srml: don't initialize genesis authorities twice

* aura: don't reinitialize genesis authorities

* aura: fix runtime_io dependency

* Bump runtime