Use stream buffering in report collector (#1504) #5635

Job	Run time
Release builds and tests	11m 54s
Basic Checks	20m 15s
Measure coverage	31m 30s
Additional Builds and Concurrency Tests	17m 5s
miri	4m 41s
sanitize (address)	19m 21s
sanitize (address, multi-threading)	20m 45s
Slow tests	22m 35s
sanitize (leak)	9m 24s
sanitize (leak, multi-threading)	9m 42s
	2h 47m 12s

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