Use stream buffering in report collector #1091

Job	Run time
Basic Checks	19m 50s
Additional Builds and Concurrency Tests	16m 24s
Measure coverage	27m 37s
miri	3m 8s
Release builds and tests	12m 7s
Slow tests	20m 41s
sanitize (address)	18m 20s
sanitize (address, multi-threading)	19m 51s
sanitize (leak)	3m 19s
sanitize (leak, multi-threading)	3m 18s
	2h 24m 35s

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