TLS reloading encounter NPE while testing with multithread

[iosmanthus]

Bug Report

1. Describe the bug

TLS reloading will produce NPE while getChannel with multiple threads.

2. Minimal reproduce step (Required)

    // Test getChannel concurrently
    factory = new ChannelFactory(v, v, v, v, caPath, clientCertPath, clientKeyPath);

    List<Thread> tasks = new ArrayList<>(8);
    for (int i = 0; i < 8; i++) {
      ChannelFactory finalFactory = factory;
      Thread t = new Thread(() -> finalFactory.getChannel("127.0.0.1:2379", mapping));
      t.start();
      tasks.add(t);
    }

    for (Thread t : tasks) {
      t.join();
    }

3. What did you see instead (Required)

Panic

4. What did you expect to see? (Required)

Test pass

5. What are your Java Client and TiKV versions? (Required)

• Client Java: 7a123a0