Move some of the celery base support into a async sub-package.
Use different named celery queues for incoming data traffic, closes #213.

All worker processes still work on all queues. But each one of them picks
a task from one queue after the other (sans prefetching taking a couple
of tasks at once). So generally the tasks on the incoming queue can no
longer dominate and prevent all other tasks from running. The split into
incoming, insert and default makes sure that even the normal scheduled
task always have a chance of running.
Include new gauges to capture the number of rows in the measure tables.