0.9.4

• Fix rare occasions of CRLs cleaned up from the cache before references to them are removed.
• Added options --once and --vrp-output filename that together make rpki-prover run only one iteration of validation and exit. Resulting VRPs will be written to the $filename. It is safe to run such one-off instance simultaneously with an instance of rpki-prover running as a server over the same cache directory.