Implement 'praefect verify' subcommand

Praefect periodically verifies the repository metadata in the background. The interval may be too long especially if there was an incident, say a disk failure. After recovering the Gitaly node, the disk may be completely empty or may contains an older snapshot which does not contain all expected repositories. In such cases, it would be great if Praefect could be manually instructed to verify the storage again as soon as possible rather than waiting for the next scheduled verification interval to pass. This commit adds the 'praefect verify' subcommand that allows for doing that. It takes in either a repository id, a virtual storage or a (virtual storage, storage) tuple and marks all replicas matching the selector as being unverified. Praefect's metadata verifier will then prioritize verifying these repositories over other repositories pending verification. This allows administrators to speed up the verification process and thus recovery.

Closes #4091 (closed)