Use stream buffering in report collector #5605

Job	Run time
Additional Builds and Concurrency Tests	14m 28s
miri	3m 4s
Basic Checks	20m 29s
Measure coverage	29m 57s
Release builds and tests	8m 20s
sanitize (address)	18m 21s
Slow tests	18m 26s
sanitize (address, multi-threading)	19m 15s
sanitize (leak)	8m 20s
sanitize (leak, multi-threading)	10m 22s
	2h 31m 2s

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