[SPARK-4138][SPARK-4139] Improve dynamic allocation settings

This should be merged after #2746 (SPARK-3795).

**SPARK-4138**. If the user sets both the number of executors and `spark.dynamicAllocation.enabled`, we should throw an exception.

**SPARK-4139**. If the user sets `spark.dynamicAllocation.enabled`, we should use the max number of executors as the starting number of executors because the first job is likely to run immediately after application startup. If the latter is not set, throw an exception.