expose the last relay chain block number as an API from parachain-system (#1761)

re:
https://forum.polkadot.network/t/blocknumber-vs-timestamps-should-we-abandon-blocktimes-altogether/4077

This exposes the `LastRelayChainBlockNumber` storage member of
`cumulus-pallet-parachain-system` with a getter and alters the behavior
of this storage item to only be updated in `on_finalize` to ensure a
consistent value throughout `on_initialize` and within transactions.

Parachains, especially with features such as asynchronous backing and
agile coretime, should not use the parachain block number as a clock.
Any feature of Polkadot intended to optimize core utilization and
parachain coretime consumption is likely to worsen this clock as it is
practically applied.