Use the check state index when filtering for passing services

This is a super common operation, but we do the filtering up on the agent. We should change the RPC logic to push that filtering down to the servers, since we already maintain an index:

https://groups.google.com/d/msgid/consul-tool/CABJfL5jZrvwNp%2BXWg56OkJ26REz%2B%3Dc1ZV_U6yv%3D7Ep%2Bs3dX%2BEA%40mail.gmail.com?utm_medium=email&utm_source=footer

Start by tracing the RPC logic that currently filters passing services on the agent, then locate the existing check state index on the servers. The work is done when matching services are filtered server-side through that index and the agent no longer performs the common filtering operation.

Written by the indexing model from the issue text.