[timestream] ensure item scanning is not starved #4806

Job	Run time
lint / lint	15s
lint / lint	15s
docs / Build linux64	1m 7s
docs / Build linux64	1m 7s
test / Windows MSVC / Build win64	3m 59s
test / Windows MSVC / Build win64	3m 59s
test / Linux gcc-12 / Build linux64	1m 28s
test / Linux gcc-12 / Build linux64	1m 28s
test / Linux gcc-10 / Build linux64	1m 16s
test / Linux gcc-10 / Build linux64	1m 16s
test / Run Linux test suite / Test (ubuntu, gcc-10, default plugins, default config)	57s
test / Run Linux test suite / Test (ubuntu, gcc-10, default plugins, default config)	57s
test / Run Linux test suite (gcc-12, all plugins) / Test (ubuntu, gcc-12, all plugins, default config)	10m 45s
test / Run Linux test suite (gcc-12, all plugins) / Test (ubuntu, gcc-12, all plugins, default config)	41s
test / Run Windows test suite / Test (windows, msvc, default plugins, default config)	59s
test / Run Windows test suite / Test (windows, msvc, default plugins, default config)	59s
test / Run Windows test suite (empty config) / Test (windows, msvc, default plugins, empty config)	56s
test / Run Windows test suite (empty config) / Test (windows, msvc, default plugins, empty config)	56s
	33m 20s

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