MB-100 some cluster_compat_mode improvements

- check_is_progress_tracking_supported relies on heartbeat
  instead of rpc
- split_live_nodes_by_version returns only ns_server nodes

Change-Id: I0852424bbe192b5f2ae84da0014f9e6deb16c14a
Reviewed-on: http://review.couchbase.org/29443
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>