Index_num_docs_pending statistic does not provide the number of pending KV items

Steps to reproduce:
1)Load 20M items in the
2)Create 5 indexes and build them concurrently.
3)Collect the index_num_docs_pending statistic. It will be seen that the stat is really provided
the count of index items remaining to be received.
4)This is misleading as the name indicates it is tracking documents when in reality it is tracking index items. We should fix either the name or the behavior.

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=secondary_400-2097_b7d_build_incrindex#secondary_400-2097_b7d_build_incrindexsecondary_400-2097_b7dbucket-1index_num_docs_pending

http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=secondary_400-2097_b7d_build_incrindex