Revocation should be retried with ACK given from agent

[lkatalin]

Currently each revocation notice is sent only once, and this could pose a problem for nodes that are either offline for that period or are still going through setup. It seems better to get an ACK from each node that it received the revocation notice, and have some mechanism to try resending it if it is not acknowledged.

Do we know that there isn't currently a mechanism in Keylime for this type of retry?

@lukehinds This one came from our conversation.