Multi-channel contexts

This extension could offer a generic cache parameters API. This ensures centralized management and avoids re-implementations in various context types. The purpose of this is to allow context types to easily add additional cache parameters which increase uniqueness of cached page versions.

The API for adding cache parameters could be put into Tx_Contexts_Context_Abstract and evaluated by Tx_Contexts_Context_Container. It should then be quite easy to extend the hash base creation in Tx_Contexts_Service_Page.