MB-8454 Improve indexing progress stats update

Instead of updating indexing progress stats (`http://localhost:5984/_active_tasks`)
after each batch write to btrees, update the stats simultanously while items
are being loaded to Mapper Queue. Hence, it will produce an incremental
progress rather than abruptly moving by batch progress.

Change-Id: Ib82260aaa17819efb9a5b51f75a93ff40df8f6b0
Reviewed-on: http://review.couchbase.org/30201
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>