Implement the "drain" feature for subscriptions and connections #4483

Job	Run time
check format (ubuntu-latest / nightly)	20s
check feature flags (ubuntu-latest / stable)	16m 31s
check examples (ubuntu-latest / stable)	54s
check docs (ubuntu-latest / stable)	58s
check licenses (ubuntu-latest / stable)	25s
check linter (ubuntu-latest / stable)	1m 33s
check minimal versions of transitive dependencies (ubuntu-latest / stable)	45s
check minimal supported rust version (ubuntu-latest / msrv)	24s
check spelling (ubuntu-latest / nightly)	6m 8s
test (ubuntu-latest / stable)	7m 47s
test (macos-latest / stable)	8m 25s
test (windows-latest / stable)	7m 4s
	51m 14s

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