Implements most of
https://github.com/paritytech/polkadot-sdk/issues/1797

Core sharing (two parachains or more marachains scheduled on the same
core with the same `PartsOf57600` value) was not working correctly. The
expected behaviour is to have Backed and Included event in each block
for the paras sharing the core and the paras should take turns. E.g. for
two cores we expect: Backed(a); Included(a)+Backed(b);
Included(b)+Backed(a); etc. Instead of this each block contains just one
event and there are a lot of gaps (blocks w/o events) during the
session.

Core sharing should also work when collators are building collations
ahead of time

TODOs:

- [x] Add a zombienet test verifying that the behaviour mentioned above
works.
- [x] prdoc

---------

Co-authored-by: alindima <alin@parity.io>