libflux: refactor reactor/watcher implementation #9991

Job	Run time
validate commits	9s
python linting	27s
Generate build matrix	8s
spelling	5s
bookworm - test-install	12m 10s
bookworm - 32 bit	12m 47s
noble - test-install	12m 32s
el9 - test-install	12m 11s
alpine - test-install	12m 50s
fedora40 - test-install	11m 47s
jammy - test-install	11m 48s
focal - py3.8	16m 59s
bookworm - gcc-12,content-s3,distcheck	19m 36s
fedora40 - clang-18	16m 49s
coverage	24m 27s
el8 - ascii	19m 27s
el8 - test-install	13m 25s
el8 - system,coverage	7m 28s
inception	21m 43s
flux-sched check	6m 57s
flux-pmix check	13m 16s
macos build only	2m 26s
flux-accounting check	4m 36s
flux-pam check	4m 23s
Generate docker manifest	1s
	4h 18m 27s

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