lib: mgmtd: remove abstraction layer and other cleanup

lib: mgmtd: remove abstraction layer and other cleanup

Code is no longer using a global FE "client context", and instead
creates client objects, rename the structure and it's uses to reflect this.

Remove an obfuscating abstraction layer whose existence was entirely
based on using a uintptr_t rather than a pointer to an declared-only struct.

Change multi-duty "params" structure into a single duty callbacks one.

Remove unsupported API code.

Signed-off-by: Christian Hopps <chopps@labn.net>