Enable implicit creation of filters.

Currently a filter must always be explicitly created before it can be used.

[MyFilter]

type=filter

module=xyzfilter

...

[MyService]

type=service

...

filters=MyFilter

That's unavoidable, if the filter has parameters that need to be set, but if it has not, it is in principle an unnecessary extra step.

By making it possible to refer directly to the filter module, in cases where the filter takes no parameters that extra step could be avoided. E.g.

[MyService]

type=service

...

filters=xyzfilter

When this is possible, it would make sense to create very specific filters that do one thing, and one thing only. For instance, MXS-5354 will make it possible to allow only certain types of queries to go through and in desired the behaviour must be configured.

With the functionality enabled by this feature, it would make sense to create specializations of it, say, readonly, writeonly, noddl, etc. that require no configuration and thus then easily could be used. E.g

[MyService]

type=service

...

filters=readonly