Fixed issue with function specification.

Prevented the module from compiling.
Created helper functions ydb_plan_node:relegate/{2,3}.

Hides the implementation details of upon what forms of requests are
the messages passed to the delegate function. Intend to change the
tuple to {relegate, Message, Extras} and remove delegate/2 as a
callback.
Switched to use ydb_plan_node:relegate/2.
Changed message from delegate to relegate.