Support Audit Filtering and Statement Auditing for N1QL

[Note: this ticket was retitled & the description changed to make it more accurately capture the work to be done in ns_server.]

There are multiple pieces to this work. 

1. Support of REST API to get information about the configuration of audit events and to enable and disable these events (and to forward this information to the audit daemon and to allow query to find out about this configuration)
2. Support of REST API to set a global blacklist filter of users that should not be audited - and to allow this blacklist be associated with each audit event (or not - in which case no filtering is done for that event)

Link to memcached audit doc: http://src.couchbase.org/source/xref/vulcan/kv_engine/auditd/README.md
Link to design doc: https://docs.google.com/document/d/18T9UquZOqz0acQycGyXZ9wh7DBq5nMoo900Tz_mrzl0