cbc-n1qlback should have an option to enforce prepared/adhoc queries

In the query file for cbc-n1qlback each line might have optional "n1qlback": property, where currently only one option accepted: boolean field "prepare". For example:

{"statement":"SELECT * FROM `travel-sample` WHERE type=$type LIMIT 10", "$type":"airline", "n1qlback": {"prepare": true}}

The "n1qlback" stripped inside cbc-n1qlback and does not become part of the query payload.