collator-protocol: Handle unknown validator heads (#5538)

There is a race condition when a validator sends its heads to the
collator, but the collator doesn't yet know these heads. Before it is
aware of these heads by importing the block(s), any collation registered
on the collator is not announced to the validators. The collations
aren't advertised, because the collator doesn't know yet that these
heads of the validator are descendants of the collations relay parent.

The solution is to store these unknown heads of the validators and to
handle them when the collator updates its own view.