Improve pixel data log handling

See:
 * https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7593&p=14636#p14636
 * http://trac.openmicroscopy.org.uk/ome/ticket/12526
 * https://trac.openmicroscopy.org.uk/ome/ticket/12538

A number of issues were reported by @PaulVanSchayck regarding delayed and skipped pixel data generation in the above thread. Until a proper message queue is in place (/cc @jballanc), these fixes should simplify how pixel data events are processed at the cost of no longer preventing one user from flooding the queue.

(Testing instructions to come)