envelope: should export a datatype for the envelope access logs

[stephen-soltesz]

Successful access to the envelop should be logged.

Mismatch between envelope access and measurement service could be a sign of a problem.

This is especially important if the inner service has no logging of its own.

Since clients create a second TCP connection between the envelope and the inner measurement service, there may be no way to associate an envelope connection with a measurement other than time. Currently there is a limit of one connection at a time, so there is no risk of concurrency.

[pboothe]

Users may provide extra data they wish to link against in their locate requests, and this is the only chance for us to put that in a DB.