Add sensitive-client key flag

[themightychris]

Add a binary or integer flag to keys that can indicate clients with network path sensitivities we want to pay attention to

A health page could take a timestamp as input and show which clients with sensitivities have and have not made successful requests to Gatekeeper since that timestamp.

If the flag is an integer, it could be a number of minutes threshold for how often we expect to see a request, and this could be used to generate ongoing alerts when a client drops off