MaxScale should detect conflicting configurations and refuse to start

MaxScale should detect if two or more instances of MaxScale are running on the same computer and refuse to start if they are started in a way that causes conflicts between them.

Case in point; MXS-1661 describes a situation where when there was an authentication error, MaxScale would log database is locked. The reason was that two MaxScale instances were running; both using the same cache directory and thus the same sqlite user database.

When a second MaxScale instances is started, it should detect if there is such a conflict, log a descriptive error message, and refuse to start.