Dispose source when no publishers. #1550

Job	Run time
multiple-arch-armv7	5m 35s
cygwin64-cache	3m 37s
multiple-arch-aarch64	5m 27s
utest-regression-blackbox-test	3m 56s
cygwin64	6m 54s
fast	1s
build-centos7	2m 11s
build-ubuntu16	1m 25s
build-cross-aarch64	2m 11s
build-cross-arm	2m 12s
build-ubuntu18	1m 33s
build-ubuntu20	1m 42s
coverage	2m 24s
multiple-arch-amd64	1m 9s
test-done	0s
	40m 17s

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