MB-9919: Provide DB data size and file size stats

This change provides DB data size and file size per vbucket
and their aggregated values through stats call.
stats "vbucket-details vbucket_id" returns the list of a given
vbucket's stats including its DB data size and file size.
For example, "vbucket-details 0" includes the following stats
for vbucket 0:
        1) vb_0:db_data_size --> DB data size for vbucket 0
        2) vb_0:db_file_size --> DB file size for vbucket 0

On the other hand, stats "all" includes the aggregated DB data
size and file size:
        1) ep_db_data_size --> DB data size aggreaged across
                               all vbuckets.
        2) ep_db_file_size --> DB file size aggregated across
                              all vbuckets.

A test case is added to ensure the value ticks up correctly.

Change-Id: Id8ae17ae7726f9a003a3245defc2ccf4ee89b77c
Reviewed-on: http://review.couchbase.org/32533
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>