Feature request, optionally return indexed values with keys

When a 2i query's results are a list of keys. This change adds
an optional request parameter to pb and http that, when set, will
cause pairs of `key:index_value` to be returned as the result of
a 2i query.

Has no effect on MapReduce. Uses capabilities for rolling upgrade
situations.
Support 2i index queries that return values for PB

Fix bug that hased the {key, value} as a key in the coverage filter
leading to incorrectly filtered results.
Merge remote-tracking branch 'remotes/upstream/pt34-index-values' into returnterms
Add in support for returnterms parameter to pass abck index terms as well as key