Changed HighsTaskExecutor to explicitly handle reference counting to … #20

Job	Run time
debug (ubuntu-latest)	3m 19s
debug_all_tests (ubuntu-latest)	3m 20s
debug64 (ubuntu-latest)	3m 21s
debug64_all_tests (ubuntu-latest)	3m 23s
release (ubuntu-latest)	3m 26s
release_all_tests (ubuntu-latest)	3m 18s
Release64 (ubuntu-latest)	3m 20s
Release64_all_tests (ubuntu-latest)	3m 20s
	26m 47s

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