[BUG] JVM does not exit after this PR https://github.com/tikv/client-java/pull/198

[marsishandsome]

Describe the bug

JVM does not exit after this PR #198

What did you do

import org.tikv.common.TiConfiguration;
import org.tikv.common.TiSession;
import org.tikv.raw.RawKVClient;
import org.tikv.shade.com.google.protobuf.ByteString;

TiConfiguration conf = TiConfiguration.createRawDefault("127.0.0.1:2379");
TiSession session = TiSession.create(conf);
RawKVClient client = session.createRawClient();
// write (k1, v1) with ttl=30 seconds
client.put(ByteString.copyFromUtf8("k1"), ByteString.copyFromUtf8("v1"), 30);

// write (k2, v2) without ttl
client.put(ByteString.copyFromUtf8("k2"), ByteString.copyFromUtf8("v2"));

client.close();
session.close();

What do you expect

What happens instead

Java Client and TiDB/TiKV version info