MB-11362: Add dcp and dcpagg stats

Add dcp and dcp agg stats that effectively point to upr and upragg
stats respectively. upr and upragg will be deprecated once all the
clients of those stats move to dcp

Change-Id: Id77301e62c23215ff97f6ee06eba82f5f64b4652
Reviewed-on: http://review.couchbase.org/38854
Reviewed-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Sriram Ganesan <sriram@couchbase.com>