Use a custom allocator for temporary buffers in reduction_ops.cc (#2775)

* port the mimalloc allocator

* hook mimalloc opt into common.h and reduction ops

* repurpose USE_MIMALLOC to only denote subbing in of default allocator with mimalloc and some refactoring

* fix unintended cherry pick diffs

* polish alloctor_mimalloc

* explicitly disable mimalloc where it already had been disabled

* update mimalloc to pull in stl allocator

* switch mimalloc stl allocator to use mimalloc library version

* turn mimalloc on by default (only the stl changes are enabled, the python interacting ones are off already and shall remain so)

* move FastAllocVector into cpu specific code

* separate out defines into arena and stl changes

* the rest of the define renames

* bfc arena allocator

* some typos and rename the bfc arena allocator to fit existing class naming conventions

* adjustments in response to comments

* different template instantiations are friends