Put task IDs in log events

With this, taskId is still 0 in two cases I know of: 1) logging from the "engine" (a command I suppose) with no task active 2) more than one task is running at once, and one of them logs from a thread it spawned itself.
In the Activator/IDE UIs we'll just have to have somewhere we display these "orphaned" log messages.
In case 1), we could probably tag the log events with an execution ID though... but it means putting two IDs in each log event and maybe it isn't useful anyway since you can determine execution ID by just seeing which ExecutionStarting/ExecutionSuccess pair the log event is in between.