Avoid race conditions when the same policy is being pulled

[flavio]

Pulling the same policy from different processes (eg: policy-server instances writing to the same directory, multiple instances of kwctl,...) can result in race conditions.

We should create a file lock inside of the policy store to ensure that:

• Given N processes that want to download registry://foo.bar/policy:1.2,
• Only one process downloads the policy to disk, all the other processes print a Waiting message and then proceed with their operation

We basically have to do something like cargo build does when invoked inside of the same directory by multiple processes at the same time.