Improve job-cmd output refreshing

[uakfdotb]

Currently if job command has a lot of output, since the front-end interface will keep refreshing the whole output every 5 seconds, it will cause browser to be slow or at least eat up lots of bandwidth.

1. Update the refresh interval to be 5 seconds between each function call instead of every 5 seconds. This should be done for most of our other setInterval calls too.
2. After initial load of the output, only fetch new lines. Or the last few lines. Maybe initial load should also be limited to e.g. 1000 lines. We don't want to store so many lines in the database anyway.