Part of https://github.com/paritytech/polkadot-sdk/issues/5079.

Removes all usage of the static async backing params, replacing them
with dynamically computed equivalent values (based on the claim queue
and scheduling lookahead).

Adds a new runtime API for querying the scheduling lookahead value. If
not present, falls back to 3 (the default value that is backwards
compatible with values we have on production networks for
allowed_ancestry_len)

Also resolves most of
https://github.com/paritytech/polkadot-sdk/issues/4447, removing code
that handles async backing not yet being enabled.
While doing this, I removed the support for collation protocol version 1
on collators, as it only worked for leaves not supporting async backing
(which are none).
I also unhooked the legacy v1 statement-distribution (for the same
reason as above). That subsystem is basically dead code now, so I had to
remove some of its tests as they would no longer pass (since the
subsystem no longer sen...