RUBY-560 fixing issue with check_connection_health with auth enabled

Calls from `ShardingPoolManager#check_connection_health` to `client['config']['mongos']` were failing when auth was enabled. Access to this collection requries an authenticated admin user.

In addition, the previous approach of comparing the seed list to the contents of the `mongos` collection was fundamentally flawed since MongoDB keeps every `mongos` that has ever connected to the cluster in that collection (even long gone, inactive `mongos` instances).

We're now simply calling `isMaster()` on each mongos to confirm that 1) it is active and reachable and 2) it is indeed a `mongos`. If either of these conditions are not true, we set `@refresh_required` to true and call close on the member.