MB-8341 Use binary names for all lazily computed stats.

Previously we assumed these stats' names to be atoms. That prevented
XDCR stats from being displayed correctly in 'by server' mode. This
happened because XDCR stats used binary names. From now all the lazily
computed stats must use binary names.

Change-Id: I5cf1022314c9e025ff1fa3f9d3143dc9a04a4156
Reviewed-on: http://review.couchbase.org/26564
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>