Label membership is now stored in the label_membership table. This is
done in preparation for adding "manual" labels, as previously label
membership was associated directly with label query executions.
Label queries are now all executed at the same time, rather than on
separate intervals. This simplifies the calculation of which distributed
queries need to be run when a host checks in.
Due to recreating the 'All Hosts' label in #1282, we get inconsistent counts
for hosts that have not checked in since that migration. This seems acceptable
for other labels, but it is important that 'All Hosts' really includes all the
hosts.
This migration adds all the hosts into that label.
Fixes#1329
