Merge pull request #1502 from andyleiserson/jemalloc #5608

Job	Run time
Additional Builds and Concurrency Tests	15m 48s
Basic Checks	26m 17s
Release builds and tests	18m 17s
Measure coverage	31m 7s
miri	5m 13s
sanitize (address)	19m 39s
Slow tests	22m 17s
sanitize (address, multi-threading)	20m 37s
sanitize (leak)	9m 57s
sanitize (leak, multi-threading)	10m 46s
	2h 59m 58s

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