Upgrade jemalloc to 0.6; add jemalloc feature #5601

Job	Run time
Slow tests	19m 17s
Additional Builds and Concurrency Tests	17m 37s
Basic Checks	21m 2s
miri	4m 37s
Measure coverage	29m 49s
Release builds and tests	10m 49s
sanitize (address)	17m 52s
sanitize (address, multi-threading)	21m 6s
sanitize (leak)	7m 45s
sanitize (leak, multi-threading)	8m 24s
	2h 38m 18s

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