Enable fork_join_executor to handle return values from scheduled functions

[hkaiser]

@gonidelis this should resolve your issue with using fork_join_executor with the transform_reduce algorithm.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
-0.03%	94.89%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (3072a57)	192571	163913	85.12%
Head commit (5561829)	188987 (-3584)	160795 (-3118)	85.08% (-0.03%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6383)	137	130	94.89%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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