PVF: drop backing jobs if it is too late (#5616)

Fixes https://github.com/paritytech/polkadot-sdk/issues/5530

This PR introduces the removal of backing jobs that have been back
pressured for longer than `allowedAncestryLen`, as these candidates are
no longer viable.

It is reasonable to expect a result for a backing job execution within
`allowedAncestryLen` blocks. Therefore, we set the job TTL as a relay
block number and synchronize the validation host by sending activated
leaves.

---------

Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>