(#217) PrevNext - Remove references to entity_table and entity_id2 from service
The PrevNext service/interface aims to be a replaceable component for use the search-caching (but not deduping). The interface that we produced from refactoring includes several references to `entity_table` and `entity_id2` -- these values are part of the SQL table, and they're needed for dedupe, but they don't seem to convey anything meaningful for search-caching. Including these fields makes the interface more complicated -- which will make it hard to implement other variants. The general gist of this commit is that we no longer fill those two columns, and we no longer read them. In a couple functions, we split the new OOP implementation (`CRM_Core_PrevNextCache_Sql`) from the traditional static BAO implementation (`CRM_Core_BAO_PrevNext`) so that we can omit these fields.
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