Executor should use all available cores by default

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Ballista default performance is poor because we default to 4 concurrent tasks. This can give a bad impression to someone comparing to other engines like Spark that use all available cores by default.

Describe the solution you'd like
I would like to default to all available cores instead. This would match the behavior in Spark.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[yahoNanJing]

Agree.