ISPN-4357 Use a specific cache for managing protofiles

Also upgrade to protobuf-2.6.0 and protostream-2.0.0.Alpha3

Jira: https://issues.jboss.org/browse/ISPN-4357
        https://issues.jboss.org/browse/ISPN-4756
That actually invokes next (in CommandInterceptor.handleDefault), but to make it clearer I'll just invoke next here rather than going via super.