MB-7354: Use ep_meta_data_memory to monitor meta data usage

This fixes the stats in the admin console and also makes sure the
alert is properly throw if meta data usage gets too high.

Change-Id: I47685928aacecf564c2b02e90da86bbae316886f
Reviewed-on: http://review.couchbase.org/23619
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>