[C++] Always prefer mimalloc over jemalloc

[pitrou]

Describe the enhancement requested

As discussed on the mailing-list, we should switch the default memory pool to mimalloc for all platforms. This would have several desirable effects:

• less variability between platforms
• mimalloc generally has a nicer, more consistent API and is easier to
  work with (in particular, jemalloc's configuration scheme is slightly
  abtruse)
• potentially better performance, or at least not significantly worse,
  than the statu quo

Component(s)

C++

[pitrou]

Issue resolved by pull request 40875
#40875

[pitrou]

For posterity: another reason to prefer mimalloc is jemalloc/jemalloc#467 (probably the cause for #44342)