Get rid of using legacydata.RequestHandler in HTTPServer, /api/tsdb/query and pkg/expr
with the goal of deprecating /api/tsdb/query and remove it completely eventually. This is
the first step of cleaning up the HTTP API query endpoint.
Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Alexander Emelin <frvzmb@gmail.com>