This reduces the query time by an order of magnitude, since it is able
to switch from a raw `stream_id` index to an index over all of
`realm_id, property, end_time`.
Instead of having a "realm.is_zephyr_mirror_realm" check for every
get_streams_traffic call, this commit udpates get_streams_traffic to
accept realm as parameter and return "None" for zephyr mirror realm.
Anders Kaseorg
Alex Vandiver
Sahil Batra
Steve Howell