Statistics for secondary index.

Patchset 1:
- added statistics for adminport.

Patchset 2:
- register reference to statistics for individual components.
- retrieve them via adminport.

Patchset 3:
- added plural form for retrieving the statistics.

Change-Id: I73da2271aeec36e49bd37a190ff37e206906cba5
Reviewed-on: http://review.couchbase.org/38138
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>