LF MPMC queue improvements

Small improvements of the LF MPMC queue

Making the LF MPMC queue generic and in a seperate header file

platform_windows.c

@@ -1,4 +1,3 @@
-#include "arena.h"
 #include "platform.h"
 
 // ----------------------------- Globals ------------------------------------
@@ -475,7 +474,7 @@ int main(int argc, char **argv) {
   arena_free(&gp_arena, (u8 **)&buf, len);
 
   // Add some extra threads to overlap I/O more aggressively
-  size_t num_threads = hw_threads * 2;
+  u8 num_threads = hw_threads * 2;
   if (num_threads < 2)
     num_threads = 2;
 
@@ -527,9 +526,7 @@ int main(int argc, char **argv) {
 
   WaitForMultipleObjects((DWORD)scan_threads, scan_tids, TRUE, INFINITE);
 
-  for (size_t i = 0; i < num_threads; i++) {
-    mpmc_push(&g_file_queue, NULL);
-  }
+  mpmc_producers_finished(&g_file_queue, num_threads);
 
   atomic_store(&g_scan_done, 1);