Allow limiting the key history proof to TOP(N) updates

[Sean Lawlor]

In practicality, it's often unnecessary to have more than a limited (e.g. 5) updates to protect against various attacks. An insertion attack by the server for example requires only 3 updates in the history to detect. Allowing some filtering option here (e.g. only retrieve latest <= N updates) greatly reduces communications and db latency for node retrieval.