The query killing functionality depends on the ResponseWriter exposing a
CloseNotify method. Since we wrap the http.ResponseWriter, the new
struct does not have that method and the HTTP handler would skip past
calling that method.

Instead of duplicating `Flush()` and `CloseNotify()` for every response
formatter, we will unify all of that under a single struct and create
formatters instead.

Also, fixes a bug where the header information from a query would not be
returned until some other data was returned with it because of
buffering and another bug in the gzipResponseWriter that wouldn't flush
the actual underlying ResponseWriter.

Previously, we implicitly added a newline and had to add one to the
number of bytes transmitted because we added that byte. That was removed
at some point and the metric was not updated to record the correct
value.