MB-10775: Implement all needed upragg stats for use in UI

Implement the following upr stats
(i)   Number of senders: producer count
(ii)  Number of items sent per producer
(iii) Number of items remaining per producer
(iv)  Total bytes sent per producer

Change-Id: I1c5e41ebabd6ccd80008b96545f69762f008c559
Reviewed-on: http://review.couchbase.org/36403
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>