Use stream buffering in report collector #5611

Job	Run time
Basic Checks	21m 39s
miri	4m 4s
Additional Builds and Concurrency Tests	16m 26s
Measure coverage	28m 17s
Release builds and tests	12m 38s
sanitize (address)	19m 40s
Slow tests	22m 6s
sanitize (address, multi-threading)	20m 43s
sanitize (leak)	9m 41s
sanitize (leak, multi-threading)	9m 59s
	2h 45m 13s

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