Detect and properly respond to subtler RRDP session desynchronization

[ydahhrk]

Per RFC 8182, FORT relies on simple checks on the RRDP session and serial to detect session desynchronization. Subtler session desynchronization could be pinpointed by keeping track of the hashes of the recently applied deltas.

This is a protection against incorrectly implemented RRDP servers, not malicious ones. As far as I can tell, the problem is unlikely.

See draft-ietf-sidrops-rrdp-desynchronization for more details.

Currently in development in the draft-spaghetti-sidrops-rrdp-desynchronization branch.