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ARROW-6478: [C++] Revert to jemalloc stable-4 until we understand 5.2.x performance issues

I propose we revert to the jemalloc 4.x stable branch where we were before until we understand how to get the performance we need for our workloads from 5.x. See upstream issue https://github.com/jemalloc/jemalloc/issues/1621

Closes #5297 from wesm/ARROW-6417-jemalloc and squashes the following commits:

eee3fbc7e <Wes McKinney> Add jemalloc pinned version to versions.txt
a68f1a3a2 <Wes McKinney> Revert to jemalloc stable-4

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>