Add an interface to register clearCache callbacks in roundupdb. Sometimes complicated computations may require an application cache. This application can now register a callback to clear the application cache, because roundup knows better when to clear it (usually when a transaction ends, either with rollback or with commit). The interface for this is currently considered experimental. The current interface is registerClearCacheCallback(self, method, param) where method is called with param as the only parameter.

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