MB-100 fix for one more cluster_compat_mode related crash

- ns_config_ets_dup is still raceful (needs to be addressed
  separately)
- cluster_compat_version key is also raceful at the initial server
  start
- cluster_compat_version value is defaulted to 'undefined'
  which crashes ++ operator if called too early

Change-Id: Ied9a6ce2eae1b5ccad922d90a3ea4ef81dc50dfb
Reviewed-on: http://review.couchbase.org/30369
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>