Elastic scaling: cumulus node + parachain system changes

[sandreim]

On the node side:

• Use v2 receipts, guarded by some rust feature or relay chain feature bit
• Compute core index for receipts
• expose claim queue offset value from prachain runtime
• slot based collator: build on cores selected by runtime via UMPSignal

In the runtime we need to make some changes to the parachain-system pallet. It needs to send the UMPSignal upward message: