Implement 'Shadow Server' data validation

[daveschaefer]

The original Perspectives paper includes the feature of validity checks and data redundancy to "[limit] the power of a compromised or otherwise malicious notary server"[1]. In this setup, each notary acts as a 'shadow server' for several other notaries, storing a record of their observations. We could implement this to help guard against data changes or corruption.

[1] Section 5: "Detecting Malicious Notaries" http://perspectivessecurity.files.wordpress.com/2011/07/perspectives_usenix08.pdf