OSX debugging

Nobody has claimed this yet.

The problem does not occur on linux.

https://github.com/microsoft/mimalloc/blob/master/src/alloc.c#L212

```
(lldb) r
Process 93543 launched: './bin/arangodbtests' (x86_64)
mimalloc: option 'secure': 0
mimalloc: option 'pool_commit': 0
mimalloc: process init: 0x7fffc980c3c0
mimalloc: debug level : 1
mimalloc: option 'show_errors': 1
mimalloc: error: trying to mi_free a pointer that does not point to a valid heap space: 0x111040000
mimalloc: assertion failed: at "/Users/jenkins/arangodb/3rdParty/mimalloc/src/options.c":121, _mi_error_message
assertion: "false"
Process 93543 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
frame #0: 0x00007fffc0a21d42 libsystem_kernel.dylib`__pthread_kill + 10
libsystem_kernel.dylib`__pthread_kill:
-> 0x7fffc0a21d42 <+10>: jae 0x7fffc0a21d4c ; <+20>
0x7fffc0a21d44 <+12>: movq %rax, %rdi
0x7fffc0a21d47 <+15>: jmp 0x7fffc0a1acaf ; cerror_nocancel
0x7fffc0a21d4c <+20>: retq
Target 0: (arangodbtests) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGABRT
* frame #0: 0x00007fffc0a21d42 libsystem_kernel.dylib`__pthread_kill + 10
frame #1: 0x00007fffc0b0f457 libsystem_pthread.dylib`pthread_kill + 90
frame #2: 0x00007fffc0987420 libsystem_c.dylib`abort + 129
frame #3: 0x00000001055e339f arangodbtests`_mi_assert_fail(assertion="false", fname="/Users/jenkins/arangodb/3rdParty/mimalloc/src/options.c", line=121, func="_mi_error_message") at options.c:136
frame #4: 0x00000001055e3e24 arangodbtests`_mi_error_message(fmt="trying to mi_free a pointer that does not point to a valid heap space: %p\n") at options.c:121
frame #5: 0x00000001055dfa53 arangodbtests`mi_free(p=0x0000000111040000) at alloc.c:212
frame #6: 0x00000001055dfbc5 arangodbtests`_ZdlPv(p=0x0000000111040000) at alloc-override.c:90
frame #7: 0x0000000105573910 arangodbtests`testing::internal::MakeAndRegisterTestInfo(test_case_name="ActiveFailover", name="creating_a_job_should_create_a_job_in_todo", type_param=0x0000000000000000, value_param=0x0000000000000000, code_location=(file = "/Users/jenkins/arangodb/tests/Agency/ActiveFailoverTest.cpp", line = 113), fixture_class_id=0x0000000109537728, set_up_tc=(arangodbtests`testing::Test::SetUpTestCase() at gtest.h:20190), tear_down_tc=(arangodbtests`testing::Test::TearDownTestCase() at gtest.h:20198), factory=0x0000000111020000)(), void (*)(), testing::internal::TestFactoryBase*) at gtest-all.cc:4053
frame #8: 0x00000001000ae474 arangodbtests`::__cxx_global_var_init.29() at ActiveFailoverTest.cpp:113
frame #9: 0x00000001000aee64 arangodbtests`_GLOBAL__sub_I_ActiveFailoverTest.cpp at ActiveFailoverTest.cpp:0
frame #10: 0x000000010f80ca1b dyld`ImageLoaderMachO::doModInitFunctions(ImageLoader::LinkContext const&) + 385
frame #11: 0x000000010f80cc1e dyld`ImageLoaderMachO::doInitialization(ImageLoader::LinkContext const&) + 40
frame #12: 0x000000010f8084aa dyld`ImageLoader::recursiveInitialization(ImageLoader::LinkContext const&, unsigned int, char const*, ImageLoader::InitializerTimingList&, ImageLoader::UninitedUpwards&) + 338
frame #13: 0x000000010f807524 dyld`ImageLoader::processInitializers(ImageLoader::LinkContext const&, unsigned int, ImageLoader::InitializerTimingList&, ImageLoader::UninitedUpwards&) + 138
frame #14: 0x000000010f8075b9 dyld`ImageLoader::runInitializers(ImageLoader::LinkContext const&, ImageLoader::InitializerTimingList&) + 75
frame #15: 0x000000010f7f947a dyld`dyld::initializeMainExecutable() + 195
frame #16: 0x000000010f7fd8c6 dyld`dyld::_main(macho_header const*, unsigned long, int, char const**, char const**, char const**, unsigned long*) + 3966
frame #17: 0x000000010f7f8249 dyld`dyldbootstrap::start(macho_header const*, int, char const**, long, macho_header const*, unsigned long*) + 470
frame #18: 0x000000010f7f8036 dyld`_dyld_start + 54
(lldb)
```
In order to reproduce you could build the tests in this branch:
https://github.com/arangodb/arangodb/tree/feature/mimalloc

Build the tests from the feature/mimalloc branch and reproduce the failure on macOS with lldb. Start at src/alloc.c:212 and alloc-override.c:90, then trace the registration path through tests/Agency/ActiveFailoverTest.cpp:113. Done means identifying why the invalid free occurs on macOS and documenting or validating a project-level fix.

Written by the indexing model from the issue text.