Use stream buffering in report collector #5634

Job	Run time
miri	4m 25s
Basic Checks	21m 4s
Measure coverage	29m 16s
Additional Builds and Concurrency Tests	15m 35s
Release builds and tests	13m 52s
sanitize (address)	17m 21s
Slow tests	20m 5s
sanitize (address, multi-threading)	19m 18s
sanitize (leak)	7m 47s
sanitize (leak, multi-threading)	9m 57s
	2h 38m 40s

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