Refactor: based: Expose cib_process_request internally

And return a legacy Pacemaker return code.

In an upcoming commit, it will be much simpler to call
cib_process_request() from another file, compared to adding special case
handling and calling cib_common_callback_worker(). It will also be
necessary to check the return code for errors.

Ref T185

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>