Process a single alert across all available threads

[mcoles]

Tried to run a test_alert this morning on our server, and it worked the first time, but not the subsequent second two times. The problem was that a single alert was processing 50 different emails, all pulling separate versions of a viz down and rendering it to PNG. The viz in question took 25+ seconds to render, so for the duration of that time, VizAlerts could not pick up new alerts to process. We should change VizAlerts from being single-threaded within a single alert, and make it use all available threads for any and all alerts. Another thing we can do, which should probably be in its own Issue, is alert VizAlerts so that it runs as a service and can scan for new alerts that come in, while it processes the existing ones.