Allow server side control of polling intervals per environment/application

[rvowles]

Currently, the polling interval is set on the client, but FeatureHub has the capability to allow per environment server side control of the polling interval.

The benefit of this is that it allows teams to set their updates (particularly for SaaS) on GET to be slow until around the time they are planning on changing their features, rolling them out for example. Then they can increase the polling interval, wait until they know everyone has that interval and then change the set of features, seeing a faster rollout of feature changes done by polling to clients like mobile and browser without the downside of significantly increased traffic to their infrastructure over a longer period of time. This also allows a much finer control of cost on SaaS.