This will enable async-backing subsystems on polkadot, the relaychain
would work in legacy mode where only candiddates built on top of
pervious relay chain are activated.

- Had to bring an unrelated change as well `minimum_backing_votes`
  because that was the v6, we can't skip versions, the value of that
  configuration on polkadot is 2, so that's what is going to be used
  after this runtime update. Changes have been running on kusama, so I
  would.

Async backing subsytems is a major change in the way collator-protocol
and  the backing work, so there are still some unknowns if this is
completely bug free. It has been running on kusama for a month
already, but not without issues:
 - Validators that did not upgrade to compatible versions will not be
   able to participate in backing, so if enough of those are randomly
   picked that group won't be able to back anything. With
   backing_group_size = 5 and minimum_backing_votes = 2, 10% validator
   not upgraded that chance is about 2.5%.

 - Additionally, same un-upgraded groups won't be able to include the
   backing votes on chain when they author blocks, so 10% of the blocks
   won't back any candidates.

 - We are still not sure if item 2) from here paritytech/polkadot-sdk#3314 (comment)
   is caused by async backing, the observable issue is sometimes after restart/upgrade
   some validators are getting 0 rewards and apparently they are not
   backing anything.

Signed-off-by: Alexandru Gheorghe <[email protected]>

Signed-off-by: Alexandru Gheorghe <[email protected]>

…polkadot

Signed-off-by: Alexandru Gheorghe <[email protected]>