Ensure relay chain block number strictly increases (#1280)

* Ensure relay chain block number strictly increases

This is a safeguard for when async backing is enabled on the relay chain and multiple parachain
blocks per relay chain block are legal. We will need to later change this to support then multiple
parachain blocks per relay chain block.

* Make the check configurable

* Fix compilation

* Update pallets/parachain-system/src/lib.rs

Co-authored-by: André Silva <[email protected]>

* Update pallets/parachain-system/src/lib.rs

Co-authored-by: André Silva <[email protected]>

* Fix test

Co-authored-by: André Silva <[email protected]>