Split scheduling goals and results to enable RAII use instead of error-prone manual release

[roberth]

Generally we count on RAII to release the locks. Unfortunately for top-level goals (and maybe some other cases), the goal object may not be destroyed in a timely manner. Ideally we would split goals and goal results so we don't need to keep goals alive until their result is needed, but that would require some big changes.

Originally posted by @edolstra in #12160 (comment)

[edolstra]

A lot of this will go away with #11927, since then stuff like RAII locks wouldn't need to be stored in the Goal object anymore but would be local variables.