MB-8187: chk_items estimate only for MUTATION/DELs

- During stat collection, account for chk_items (the estimate)
only during TAP_MUTATION and TAP_DELETIONS, and not at checkpoint
start and end messages.
- This way docsTotal and docsTransferred stats will show as zero
during a rebalance if the bucket were empty.

Change-Id: I5df5d19f16b7dd7abe7465d356bb748d56e20d8e
Reviewed-on: http://review.couchbase.org/29882
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>