Allow end-user specified classification

If the MaxScale parser fails to parse a statement, it may be incorrectly classified (e.g. a statement that should be sent to master is sent to some slave, a statement that should be sent to one server is sent to all, etc.).

Fixing these kind of issues typically requires that the MaxScale parser is modified, which at minimum requires a new MaxScale release and occasionally may be time-consuming and hard to implement.

To allow end-users and support to make quick fixes, it would be valuable if:

• it would be possible to specify a canonical statement and how it should be classified (to master, to some slave or to all)
• MaxScale would consult that user-specified classification if it fails to parse a statement.

That way a failure of MaxScale to correctly classify a statement could quickly be addressed by end-users and support as a temporary measure until a proper fix is available.