Add option to disable built-in scheduler in eclserver

In both LN and community we have a eclscheduler component, but in LN eclserver has historically taken the role of running the scheduler internally.
That means that if a separate eclscheduler and a eclserver are present in the same environment there can be multiple schedulers dealing with the same eclserver.
This can also be an issue if there are multiple load-balanced eclservers.

Add an option to disable the built-in eclserver scheduler.
This maintains backward compatibility, i.e. it will still be on by default.

In the long run, the eclscheduler should be removed from eclserver altogether, see : https://track.hpccsystems.com/browse/HPCC-18792