Make `BlockImport` and `Verifier` async (#8472)

* Make grandpa work

* Introduce `SharedData`

* Add test and fix bugs

* Switch to `SharedData`

* Make grandpa tests working

* More Babe work

* Make it async

* Fix fix

* Use `async_trait` in sc-consensus-slots

This makes the code a little bit easier to read and also expresses that
there can always only be one call at a time to `on_slot`.

* Make grandpa tests compile

* More Babe tests work

* Fix network test

* Start fixing service test

* Finish service-test

* Fix sc-consensus-aura

* Fix fix fix

* More fixes

* Make everything compile *yeah*

* Fix build when we have Rust 1.51

* Update client/consensus/common/src/shared_data.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/common/src/shared_data.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/common/src/shared_data.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/common/src/shared_data.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/common/src/shared_data.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/babe/src/tests.rs

Co-authored-by: André Silva <[email protected]>

* Update client/consensus/babe/src/tests.rs

Co-authored-by: André Silva <[email protected]>

* Fix warning

Co-authored-by: André Silva <[email protected]>