[threaddpool] Fix deadlock in thread pool with monitor thread

Because the monitor thread was not marked as background thread, remove_and_abort_threads (threads.c:2807) would mark it as needed to be waited to shutdown the runtime, and wait_for_tids (threads.c:2652) would then wait indefinitely for it to finish. The issue was that the monitor was waiting on the semaphore monitor_sem, and thus would never exit.

Thread 7 (Thread 0x1c13 of process 99776):
#0  0x00007fff86590a56 in ?? () from /usr/lib/system/libsystem_kernel.dylib
#1  0x00000001003cc03a in mono_sem_wait (sem=0x1004f7f50, alertable=0) at mono-semaphore.c:103
#2  0x00000001002cef41 in monitor_thread (unused=0x0) at threadpool.c:898
#3  0x00000001002cb78f in start_wrapper_internal (data=0x10500abf0) at threads.c:657
#4  0x00000001002cb4a1 in start_wrapper (data=0x10500abf0) at threads.c:704
#5  0x00000001003d61d4 in inner_start_thread (arg=0x7fff5fbfe500) at mono-threads-posix.c:84
#6  0x00007fff8e7c9899 in _pthread_body () from /usr/lib/system/libsystem_pthread.dylib
#7  0x00007fff8e7c972a in _pthread_start () from /usr/lib/system/libsystem_pthread.dylib
#8  0x00007fff8e7cdfc9 in thread_start () from /usr/lib/system/libsystem_pthread.dylib
#9  0x0000000000000000 in ?? ()

Thread 1 (Thread 0x1503 of process 99776):
#0  0x00007fff86594716 in ?? () from /usr/lib/system/libsystem_kernel.dylib
#1  0x00007fff8e7cbc3b in _pthread_cond_wait () from /usr/lib/system/libsystem_pthread.dylib
#2  0x000000010039cfaa in _wapi_handle_timedwait_signal_handle (handle=0xa55, timeout=0x0, alertable=1, poll=0) at handles.c:1595
#3  0x000000010039d03d in _wapi_handle_wait_signal_handle (handle=0xa55, alertable=1) at handles.c:1540
#4  0x00000001003b7ba3 in WaitForSingleObjectEx (handle=0xa55, timeout=4294967295, alertable=1) at wait.c:194
#5  0x00000001003b865d in WaitForMultipleObjectsEx (numobjects=1, handles=0x7fff5fbff1f8, waitall=1, timeout=4294967295, alertable=1) at wait.c:516
#6  0x00000001002c75df in wait_for_tids (wait=0x7fff5fbff1f8, timeout=4294967295) at threads.c:2658
#7  0x00000001002c70e5 in mono_thread_manage () at threads.c:2951
#8  0x00000001000d551b in mono_main (argc=6, argv=0x7fff5fbff9e8) at driver.c:2021
#9  0x0000000100001bd1 in mono_main_with_options (argc=6, argv=0x7fff5fbff9e8) at ./main.c:91
#10 0x00000001000017d3 in main (argc=6, argv=0x7fff5fbff9e8) at ./main.c:122