Rework startup configuration handling

At startup, MaxScale first "sniffs" the main configuration file, primarily in order to detect whether some default directories have been changed, makes initial configurations and starts the threads. Then it reads the configuration anew, this time processing additional static configuration files and runtime changes.
Now that the number of threads can be adjusted at runtime, it leads to MaxScale first creating as many threads as specified in the main configuration file and then later, if the number of threads at an earlier run was decreased, shut down threads. This is non-optimal. Things need to be changed so that no more than the desired number of threads are created at startup.