add USE INDEX hint to the n1ql DSL

There has recently (ie post N1QL DP4) been the addition of the USE INDEX clause in SELECT (and DML) statements. This hints to use a specific index.

Example:
select * from CUSTOMER USE INDEX(VPX_CUSTOMER) limit 1;

Reference for SELECT:
https://github.com/couchbase/query/blob/master/docs/n1ql-select.md