chore: Add global lock for execute_rental_request_proposal, fix guards overriding each other

[fxgst]

In addition to NNS's check that there can be only one Rental Request at a time, the SRC also enforces that there can be no concurrent execution of execute_rental_request_proposal. Also give guards different names if there are more than one to prevent them freeing each other because they get overridden.

[andrew-lee-work]

I left a comment on the new guard in execute_rental_request_proposal.

[andrew-lee-work]

It looks like guard_subnet is scoped to execute_rental_request_proposal. In that case it would make more sense to use a tag specific to the function instead of sharing a tag with the subnet locks. (Similar to how refund uses the "refund" tag which is specific to it.)

I think there should also be a lock on the subnet, so there's only one update on the subnet at a time. That would look like:

       let _guard_subnet =
            CallerGuard::new(payload.subnet_id, "nns").expect("Fatal: Concurrent call on subnet");

(Copied from below.) Perhaps that's actually what was intended here, since it's called "guard_subnet", not "guard_method".

[fxgst]

You're right, the _guard_subnet is a mistake. But the subnet_id is not available yet, so I opted for the global lock called "nns" (which makes sure that there cannot be concurrent calls to this function).