prepared statements aren't fully audited

Vulcan build 1971

Using n1qlback to run a query that consists of:

{"prepared":"airline_keys"} (where "airline_keys" was prepared manually previously)

The audit statement doesn't show the statement.  Ideally we would also get the name of the prepared query audited as well:

{"timestamp":"2018-02-23T13:49:52.410Z","real_userid":{"source":"local","user":"Administrator"},"requestId":"141a94fe-621b-4de4-9f0e-b2bdcc014e67","statement":"","isAdHoc":false,"userAgent":"libcouchbase/2.8.4 (Linux-4.14.5-300.fc27.x86_64; x86_64; GNU 4.8.5)","node":"ec2-52-36-167-40.us-west-2.compute.amazonaws.com:8091","status":"success","metrics":{"elapsedTime":"856.403µs","executionTime":"844.268µs","resultCount":1,"resultSize":210},"id":28672,"name":"SELECT statement","description":"A N1QL SELECT statement was executed"}