Using timestamps to validate revocation effective time for notary.x509 signatures

[yizha1]

Per the comment in PR #429 (comment), this issue is to track the work on using timestamps to validate revocation effective time for notary.x509 signatures.

This issue notaryproject/specifications#309 is also related.

[github-actions]

This issue is stale because it has been opened for 60 days with no activity. Remove stale label or comment. Otherwise, it will be closed in 30 days.

[github-actions]

This issue is stale because it has been opened for 60 days with no activity. Remove stale label or comment. Otherwise, it will be closed in 30 days.

[github-actions]

Issue closed due to no activity in the past 30 days.