MB-100 use ep_engine aggregated stats to calculate the stats:

 - couch_docs_data_size
 - couch_docs_disk_size

ep-engine provides the following agregated stats:

        1) ep_db_data_size --> DB data size aggreaged across
                               all vbuckets.
        2) ep_db_file_size --> DB file size aggregated across
                              all vbuckets.

couch_docs_disk_size = ep_db_file_size

couch_docs_data_size is calculated from ep_db_data_size with
the following adjustment:

    MB-5900 Fake data size when disk size is too small.

    When index or bucket disk size is too small and hence metadata
    overhead is too big, we assert that data size equals disk size. In
    such case fragmentation would be 0 which is more intuitively expected
    for the end user.

Change-Id: I0497d3d5c8526b04a5df0f19557fdc66441a7696
Reviewed-on: http://review.couchbase.org/32787
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>