Refactor node/URI selection from randomized to round robin

Currently each service contains code to randomly use a node from the global nodes list for all services which are global; for example:

https://github.com/couchbase/couchbase-net-client/blob/master/src/Couchbase/Services/Query/QueryClient.cs#L112-L117

This needs to be refactored and changed to support round robin node/URI selection and usage. Since this logic is used for nearly all "global" HTTP services, the solution should probably be done in a way to reduce code duplication an enhance reusability and testability.