bug 1600706: change update_incoming batch size from 100 to 500
bug 1600706: change update_incoming batch size from 100 to 500

This will cause a single celery task to process 500 rather than 100.
Overall, this will reduce the amount of bookkeeping done per
update_incoming item. The mean time for executing an update_incoming
task is pretty small (48ms) compared to the other tasks (10k ms), so
this seems like a safe try.

mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea