MXS-1301: function matches functionless queries in accept mode

A function type rule matches a query if the query uses a function defined
in the rule. This is the desired behavior for blacklist mode operation
with `action=block`.

When in whitelist mode, all queries must match a rule to be allowed
through. For function type rules, this fact is problematic as queries that
don't use functions are blocked. The desired behavior is to allow the use
of certain functions while preventing the use of others.

The allowed set of functions should always contain the "empty set" (no
functions are used) when the filter is in whitelist mode.

Documentation/Release-Notes/MaxScale-2.2.0-Release-Notes.md

@@ -54,6 +54,12 @@ The `match` and `exclude` parameters were changed to use PCRE2 syntax for the
 regular expressions. The regular expression should be enclosed by slashes
 e.g. `match=/select.*from.*test/`.
 
+### Dbfwfilter
+
+The `function` type rule will now match a query that does not use a function
+when the filter is in whitelist mode (`action=allow`). This means that queries
+that don't use functions are allowed though in whitelist mode.
+
 ## Dropped Features
 
 ### MaxAdmin