Make IO threads killable so that they can be canceled at any time.

This commit can be cherry picked to 6.0 only if we also cherry pick f866023.

src/networking.c

@@ -2966,6 +2966,7 @@ void *IOThreadMain(void *myid) {
     snprintf(thdname, sizeof(thdname), "io_thd_%ld", id);
     redis_set_thread_title(thdname);
     redisSetCpuAffinity(server.server_cpulist);
+    makeThreadKillable();
 
     while(1) {
         /* Wait for start */