Loans interest hook

[daniel-savu]

• Adds an on_initialize hook to the loans pallet, which accrues interest for any market that was interacted with in the previous block.
• To achieve this, a MarketToReaccrue storage item is added. It is a binary flag which is enabled whenever a user interaction occurs. It is only set to false by the on_initialize hook, so if there are user interaction in a block where interest accrual was triggered by the hook, the flag is still enabled and the hook will run again in the following block.
• The signature of accrue_interest changes because one more argument is added (the boolean flag). It could be argued that this flag could be set in a function of its own and not in accrue_interest, but since these actions are tightly related, I think the current implementation makes sense. Let me know if you think otherwise.
• Note: The hook performs unbounded iteration on all markets to find those which need to be re-accrued. Such iteration happens when calculating collateralization as well (thanks @sander2 for pointing that out) and it would be great if we could set an upper bound on the market count in the storage item itself. I asked about this on the stack exchange here.

[sander2]

Hmm I'm still not the biggest fan of this approach.. It seems quite hacky to me. There is also a usability problem with updating it only in the next block: the UI only can display the final value 12 seconds after the block got included

[sander2]

This isn't being used?

[daniel-savu]

I wanted to parameterize on_initialize on the number of markets, but since you pointed out that several extrinsics iterate over markets, it'd be cleaner to have a separate PR for this parameterization.

[sander2]

This one is easier to address though since on_initialize uses a dynamic weight. Even when we eventually set an upperbound it's still better to have on_initialize return the actual weight rather than the upper bound (especially since this is called every block)