CBD-92: use fast config access path for cluster compat version
CBD-92: use fast config access path for cluster compat version

I want to be able to use it in more performance critical places, thus
I need that faster access.

ns_config_ets_dup path is correct here because as part of cluster
compat upgrade we sync config changes.

Change-Id: I57254ebbaa91fc9adbf50874e684830c3c0b281e
Reviewed-on: http://review.couchbase.org/21518
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

membase/ns_server,nimishzynga/ns_server,membase/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,membase/ns_server,membase/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,vzasade/ns_server,mschoch/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,daverigby/ns_server,mschoch/ns_server,vzasade/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,daverigby/ns_server,vzasade/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,daverigby/ns_server,ceejatec/ns_server,vzasade/ns_server