Make the Win32 thread handles are available after the threads exit.

FossilOrigin-Name: 565c5af7a75ad5c759ce1a61dab3a61c42819644

src/os_win.c

@@ -1275,6 +1275,13 @@ void sqlite3_win32_sleep(DWORD milliseconds){
 #endif
 }
 
+DWORD sqlite3Win32Wait(HANDLE hObject){
+  DWORD rc;
+  while( (rc = osWaitForSingleObjectEx(hObject, INFINITE,
+                                       TRUE))==WAIT_IO_COMPLETION ){}
+  return rc;
+}
+
 /*
 ** Return true (non-zero) if we are running under WinNT, Win2K, WinXP,
 ** or WinCE.  Return false (zero) for Win95, Win98, or WinME.