Enhance DefaultEHCacheOAuthDataProvider to share an overflow file.

The class `org.apache.cxf.rs.security.oauth2.provider.DefaultEHCacheOAuthDataProvider` has a `diskStore` attribute for configuring the cache overflow destination. This can not be shared between CacheManagers, and JVMs.

This enhancement request is to extend `DefaultEHCacheOAuthDataProvider` so that multiple JVMs and CacheManagers can share a cache overflow file.