past-session validator discovery APIs (#2009)

* guide: fix formatting for SessionInfo module

* primitives: SessionInfo type

* punt on approval keys

* ah, revert the type alias

* session info runtime module skeleton

* update the guide

* runtime/configuration: sync with the guide

* runtime/configuration: setters for newly added fields

* runtime/configuration: set codec indexes

* runtime/configuration: update test

* primitives: fix SessionInfo definition

* runtime/session_info: initial impl

* runtime/session_info: use initializer for session handling (wip)

* runtime/session_info: mock authority discovery trait

* guide: update the initializer's order

* runtime/session_info: tests skeleton

* runtime/session_info: store n_delay_tranches in Configuration

* runtime/session_info: punt on approval keys

* runtime/session_info: add some basic tests

* Update primitives/src/v1.rs

* small fixes

* remove codec index annotation on structs

* fix off-by-one error

* validator_discovery: accept a session index

* runtime: replace validator_discovery api with session_info

* Update runtime/parachains/src/session_info.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* runtime/session_info: add a comment about missing entries

* runtime/session_info: define the keys

* util: expose connect_to_past_session_validators

* util: allow session_info requests for jobs

* runtime-api: add mock test for session_info

* collator-protocol: add session_index to test state

* util: fix error message for runtime error

* fix compilation

* fix tests after merge with master

Co-authored-by: Sergei Shulepov <sergei@parity.io>
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