Relative slots (#2820)

* Initial work on relative slots for BABE

* More work

* Update core/consensus/babe/src/lib.rs

`Aura` → `Babe`

Co-Authored-By: Pierre Krieger <[email protected]>

* More work on relative slots

* Add missing field in test-runtime

* Bump `impl_version` and `authoring_version`

* Fix compile errors and warnings

* Upgrade dependencies

* Update dependencies more

* Revert some updates to dependencies

Somehow, those broke the build

* Fix compilation errors

* `Duration` → `u128` in calculations

* `slot_duration` is in milleseconds, not seconds

* Median algorithm: ignore blocks with slot_num < sl

* Fix silly compile error

* Store a duration, rather than an instant

It is more useful

* Fix compilation errors

* `INVERSE_NANO` → `NANOS_PER_SEC`

Also: `1000_000_000` → `1_000_000_000`

Suggested-by: Niklas Adolfsson <[email protected]>

* Un-bump `authoring_version`

* Disable median algorithm when `median_required_blocks` is 0

Otherwise it would panic.

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* Simplify panic

* Fix build error

* Create `SignedDuration` struct

for signed `Duration` values.

Suggested-by: Bastian Köcher

* Refactor median algorithm into separate function

* Add issues for FIXMEs and respond to code review

* Fix minor warnings