Jobs execute one at a time

Due to XCOMMONS-400, the job manager executes only one job at a time, in sequence.
Mail Archive loading jobs being long-lived by nature, that means for example that during a loading session, it is impossible to install an extension with EM - EM job is waiting for loading job to terminate.
Currently a workaround is to stop the mail archive job in order to unlock other jobs if needed - thanksfully it will be possible from mail archive UI in 0.3.
Another possibility is to merely use the job features (like progress etc), but execute the Thread on our own (so we can execute as many as needed in parallel).