Various BaseProducer::flush fixes #690

Job	Run time
lint	2m 23s
lint	2m 23s
check (macos-11.0)	1s
check (macos-11.0)	1s
check-minimal-versions	1m 7s
check-minimal-versions	1m 7s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	5m 13s
check (windows-2019, cmake-build,libz-static,curl-static, cmake-build,libz-static,curl-static)	5m 13s
check (ubuntu-20.04, tracing)	2m 10s
check (ubuntu-20.04, tracing)	2m 10s
test (7.7.0, 3.7)	6h 0m 14s
test (7.7.0, 3.7)	6h 0m 13s
test (7.5.1, 3.6)	6h 0m 13s
test (7.5.1, 3.6)	6h 0m 13s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	5m 0s
check (ubuntu-20.04, cmake-build,ssl-vendored,gssapi-vendored,libz-static,curl-static,zstd, cmake...	5m 0s
test (7.5.1, 3.5)	2h 52m 23s
test (7.5.1, 3.5)	2h 52m 23s
	1d 6h 17m 27s

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