Allow early refund of slot deposit

[Ank4n]

closes #877
context: https://hackmd.io/4OLfULI6RtaPdE-L_hur4g

Changes since last approach (Updated on 19th Nov):

Instead of having a config value that determines how early refund can be applied before the slot expiry, now early refund can only be applied anytime during the last LP of the lease. Parachains should have at minimum MinLeasePeriodsForEarlyRefund LPs in their current lease to be able to refund early.

Summary

This change would allow a parachain to refund their slot deposit during their last lease period, given that they leased out minimum of MinLeasePeriodsForEarlyRefund in the last auction. For parachains who won the lease with a crowdloan, they could then refund crowdloan funds to contributors, and dissolve the crowdloan. This gives them opportunity to either start a new crowdloan or bid directly with their released funds.

If MinLeasePeriodsForEarlyRefund is set to 0, a slot deposit can never be refunded before it expires.

Changes

• Config const MinLeasePeriodsForEarlyRefund: Determines how many minimum lease periods a leaser in their latest lease to be eligible for early refund. This value should always be greater than 0. If set to 0, early_lease_refund is disabled.
• New signed extrinsic early_lease_refund: Only para or para manager can call this or can be executed via governance with Lease Admin origin track. It allows a parachain to unreserve their lease deposit during the last lease period of their full lease. This will unreserve the deposit of the slot but would not downgrade the parachain yet. Parachain would be downgraded as it happens today if at the start of the next lease period it does not have any new leases remaining or left alone if a new lease has become available and update the fund reserved to match the new requirement.
• Storage double map LeaseInfo: Keeps track of how much funds is reserved as well as period count of the latest lease for a leaser.

early_lease_refund can be followed by the calls refund and dissolve on the crowdloan pallet for the para_id which will remove the crowdloan and allow the parachain to create a new crowdloan for the same para_id.

TODO

• All existing tests pass in the scenario where early_lease_refund is not called. This ensures we do not impact any existing flows.
• Unit + Integration test
• Allow only Lease Admin or parachain manager to call early_lease_refund.
• Migration of LeaseInfo.
• Bench
• Only leasers who have more than one lease can early refund slot deposit.
• Intialise lease count of existing parachains to 4 (we need it to be greater than 2 to enable early_lease_refund).
• Allow para manager or para origin to call early_lease_refund.

[Ank4n]

Fungibles would fix this but probably shouldn't migrate in this PR

[Ank4n]

I guess its too late to be useful anymore? @SBalaguer

Closing this PR.