add parallel view priority and parallel execution of multiple jobs

This enables the user to define a priority on a load_balanced_view.

There are 4 levels. The levels are taken from a sql server.
Note: It doesn't block out lower priority jobs

```
PRIORITY_CRITICAL  Highest priority user
PRIORITY_HIGH      These jobs take precedences over normal jobs
PRIORITY_NORMAL    Default operation level for all jobs
PRIORITY_LOW       Lowest priority user jobs, background
                   loads, jobs that should not affect other activity
```

Example:
```python
PRIORITY = util.PRIORITY_NORMAL

c = Client(profile='default')
lview = c.load_balanced_view()
lview.set_flags(priority=PRIORITY)
```

It also executes multiple jobs in parallel. The current implementations executes all the tasks of the first job that was submitted.