Give the result event queue priority.

Processing result events sooner means that we can potentially
stop builds or at least stop launching builds sooner.

New trigger events are only likely to cause us to start new
builds (which may need to be stopped).

Change-Id: I5c33ed74cf4af84900c96c2155b3cec5252248d5
Reviewed-on: https://review.openstack.org/23117
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins