Cache updates to the set of instances registered with a VIP address or application

[seh]

Per the preceding discussion in #53, and sitting on top of #55, poll the instances for either a VIP address or application periodically, and store the latest result for safe retrieval on demand.

Here we expose a few new types and functions:

• function ScheduleAppInstanceUpdates
• type InstanceSetSource
• function NewInstanceSetSourceForVIPAddress
• function NewInstanceSetSourceForSecureVIPAddress
• function NewInstanceSetSourceForApp

The first is a way to get a channel of InstanceSetUpdate for an application, that delivers (optionally) filtered and shuffled instances instead of the whole application like NewAppSource does. That would have been part of #55 had I thought of it early enough.

[seh]

Say, @damtur, are you reviewing changes for fargo these days? If not, is there anyone else you'd recommend that I request to review this proposal?

[damtur]

Hey @seh I've had time to go through only first PR, and just to grasp a concept of all changes. It all looks well thought so I shouldn't expect big problems. You can always ping @wliao008 or @tysonstewart for CR as well. Other then that I'll try to look into those next week.

[seh]

Hey @damtur, did you see this comment and the one that followed?

I'll follow up with another PR, before we expose the current interface in a release, but I'd like your feedback on the parameter type.

[damtur]

Hey @seh Please leave it as boolean. Thanks for a follow up.