mkcollector: validate measurement on submission?

[bassosimone]

The v2.0.x spec says that the measurement top-level keys MUST be validated. The spec is actually not super clear on whether validation MUST be performed by the server and SHOULD be performed by the client, or whether validation is also a MUST for the client. A strict interpretation of the spec would suggest the latter. Also, we are talking about the top level keys.

In any case, I believe a review of the top-level keys spec is in order before doing anything in here. Additionally, the opportunity of writing this code in Golang instead seems to suggest that we don't actually want to proceed immediately with this issue.