[C++] jemalloc_set_decay_ms precedence

[asfimport]

I've noticed that the jemalloc const configuration je_arrow_malloc_conf overrides the arrow public function jemalloc_set_decay_ms().

Is their a way to call jemalloc_set_decay_ms so that it has the right precedence ?
-> if yes, I believe this should be specified in the comments
-> if no, the function should be deprecated

Reporter: Rémi Dettai / @rdettai

_{Note: This issue was originally created as ARROW-9008. Please see the migration documentation for further details.}