[Avro] Ease future customization by unifying place where a new visitor is created.

[MichalFoksa]

To ease customization by a user I suggest to add protected method AvroFormatVisitorWrapper (ex VisitorFormatWrapperImpl) createVisitorWrapper() into AvroFormatVisitorWrapperwhich creates and returns a new instance of visitor wrapper. This method would be used on each place where new visitor instance is created in ArrayVisitor, MapVisitor and RecordVisitor.

By having this in place it is easier to add / modify existing functionality by a user. He just needs to extend existing visitor wrapper and override particular method (just ask me how I know it).

```
protected AvroFormatVisitorWrapper createVisitorFormatWrapperImpl() {
    return new AvroFormatVisitorWrapper(_schemas, _provider);
}
```

[cowtowncoder]

While I am +1 for improving naming, isn't this backwards-incompatible change, and something that can't really go in 2.13 but should wait for 3.0?

[MichalFoksa]

:( OK - rename is undone

[cowtowncoder]

Ok so I think I can see why this is needed. I added a note where naming seems confusing wrt method -- I think I can see why naming of the class is unfortunate.

Once we get past that, maybe it would make sense to also file a follow-up issue about actual renaming like you suggested first. I'd have to think if it actually could go in 2.14, after all, or at least in 3.0.

[cowtowncoder]

LGTM! Will merge.

[MichalFoksa]

Thanks!