Support completer queries from SearchBackend
Support completer queries from SearchBackend

This allows the SearchBackend to return query completions for virtual queries:
e.g. virt.v1.*.servers-stat* returns ["virt.v1.*.servers-status:"].

Why test for running a completion query using 'HasSuffix(query, "*")' rather
than 'format == "completer"? The reason is that graphite-web translates
format=completer from the javascript into format=json and a standard 'find'
request, and it is this standard 'find' request which reaches carbonzipper.

This shuffles the find response encoding bits into a function so that the
search completer case can early-out.

dgryski/carbonzipper,dgryski/carbonapi