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
Since the CacheManager would provide the same cache instance, we technically don't need a lazy initialization lock, both can write to the variable concurrently just fine.  But maybe we want to leave if we change that contract of DefaultCacheManager.