Endpoints sets management

[kailuowang]

Manage endpoints by sets (each service provide an endpoint sets), each set has a version and name(maybe same as root path?).

• A whole set can be offline if all backing service instance go offline, or they no longer support such version.
• New definition of endpoint set will replace the old set with the same name and version.
• For now does not support serving multiple sets with the same name (root path) but different version.

[nsauro]

Should authentication/authorization concerns also be included so that REST-API can perform the necessary operations, and thereby keep that logic all in a single place? I think I remember that being a concern the first time this subject came up.

[kailuowang]

@nsauro yes, that's going to be addressed in the other issue #67