Support SslMasterKeyHandler in NettyServerBuilder

I have a problem with adding SslMasterKeyHandler#newWireSharkSslMasterKeyHandler on the server side. I tried to do this with channelFactory:
```
builder.channelFactory(() -> {
final ServerChannel ch = GrpcUtil.getDefaultServerChannelFactory().newChannel();
final ChannelPipeline pipeline = ch.pipeline();
pipeline.addLast(SslMasterKeyHandler.newWireSharkSslMasterKeyHandler());
return ch;
})
```
and later with ProtocolNegotiator:
```
final SslContext sslContext = GrpcSslContexts.configure(sslClientContextBuilder).build();
final io.grpc.netty.ProtocolNegotiator negotiator = ProtocolNegotiators.serverTls(sslContext);
builder.protocolNegotiator(tlsNegotiator(negotiator);
```

In the first case, the setting had no effect. In the second case, the server worked incorrectly (it stopped accepting connections, even without a handler, just with a negotiator wrapper).

What should be the proper implementation?

まず NettyServerBuilder.channelFactory と protocolNegotiator から始め、次に SslMasterKeyHandler.newWireSharkSslMasterKeyHandler と ProtocolNegotiators.serverTls を読みます。サーバー側の TLS マスターキー処理でサポートされている統合ポイントを特定します。完了の条件は、サーバーが接続を受け付けることを妨げずに、ハンドラーが NettyServerBuilder で動作することです。

索引モデルが issue の本文から書いたものです。