Improve kv::Watcher without messages #4436

Job	Run time
check examples (ubuntu-latest / stable)	50s
check feature flags (ubuntu-latest / stable)	8m 55s
check docs (ubuntu-latest / stable)	1m 2s
check format (ubuntu-latest / nightly)	12s
check licenses (ubuntu-latest / stable)	22s
check linter (ubuntu-latest / stable)	1m 28s
check minimal versions of transitive dependencies (ubuntu-latest / stable)	25s
check minimal supported rust version (ubuntu-latest / msrv)	35s
check spelling (ubuntu-latest / nightly)	5m 41s
test (ubuntu-latest / stable)	7m 56s
test (macos-latest / stable)	11m 40s
test (windows-latest / stable)	12m 42s
	51m 48s

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