Add health query for multiple services

While we are working on https://github.com/hashicorp/consul/issues/1244 we should also consider this request to retrieve multiple services - https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/consul-tool/MMlH8CIOMbQ/p8uRQDL5BAAJ.

It's worth thinking through a simple extension similar to that proposed, or if this would fit better in /v1/txn (or both).

Start by reading Consul issue #1244 and the linked Google Groups proposal, then compare the suggested extension with the /v1/txn endpoint. The issue does not identify implementation files or tests; completion would require an agreed design for retrieving health information for multiple services and deciding whether it belongs in the proposed API, /v1/txn, or both.

Written by the indexing model from the issue text.