Replace the "server api" logic in memcached with free functions

The engine interface was designed when all engines was built as shared objects where the objects got provided with a function pointer they could call to request functionality from the server.

In its design it allowed for different versions of the API (the engine would typically ask for the version it knew about; and the server should return that version or its own version if it was older than the engine). This was intended for the opensource world where people could build different engines, and we would extend the server.

We ship a complete product so our engines and core always match; in addition we link everything static so this extra complexity have no extra value and could just be replaced with free functions.