Resolves https://github.com/paritytech/polkadot-sdk/issues/4776

This will enable proper core-sharing between paras, even if one of them
is not producing blocks.

TODO:
- [x] duplicate first entry in the claim queue if the queue used to be
empty
- [x] don't back anything if at the end of the block there'll be a
session change
- [x] write migration for removing the availability core storage
- [x] update and write unit tests
- [x] prdoc
- [x] add zombienet test for synchronous backing
- [x] add zombienet test for core-sharing paras where one of them is not
producing any blocks

_Important note:_
The `ttl` and `max_availability_timeouts` fields of the
HostConfiguration are not removed in this PR, due to #64.
Adding the workaround with the storage version check for every use of
the active HostConfiguration in all runtime APIs would be insane, as
it's used in almost all runtime APIs.

So even though the ttl and max_availability_timeouts fields will now be
unused, they will remain part of the host configuration.

These will be removed in a separate PR once #64 is fixed. Tracked by
https://github.com/paritytech/polkadot-sdk/issues/6067

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>
Co-authored-by: command-bot <>