generate unique new SpecificData for every generated class for MODEL$

[radai-rosenblatt]

we currently generate

MODEL$ = SpecificData.get();

while vanilla (modern) avro generates

MODEL$ = new SpecificData();

but under 1.11.0 this can cause deadlocks over access to classcache. upstream avro might have fixed this in https://issues.apache.org/jira/browse/AVRO-3243 but we should move to unique MODEL$s if possible.

this would mean a helper method to create a new SpecificData if possibe given avro runtime version