Nova ignores delete requests while instance is in deleting state

Right now the code in compute.api delete methods ignores delete requests if the instance is already in deleting state (https://github.com/openstack/nova/blob/69ce0f01b60dfe0f020ac57eb82a42e5935064c4/nova/compute/api.py#L2257-L2262). It was result of discussion in https://bugs.launchpad.net/nova/+bug/1248563 and mailing list thread referenced there. Though right now, after python 2 EOL, it is possible to allow multiple delete requests, without having to worry about delete requests piling up waiting on the instance uuid lock, if the lock will be acquired with timeout. Python 3 supports passing timeout argument to lock.acquire, so it'll have to be a pretty easy change to oslo.concurrency to allow passing that timeout through (for example using acquire call with timeout in https://github.com/openstack/oslo.concurrency/blob/c08159119e605dea76580032ca85834d1de21d3e/oslo_concurrency/lockutils.py#L156-L162). The instance deletion flow could then use such way of lock acquisition, and if it was not acquired, to allow user to retry later.