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.

(cherry picked from commit c37acb82aa)

src/networking.c

@@ -2948,6 +2948,7 @@ 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 */