TCPClient will crash when we do 'Disconnect' and destruct the object immediately

The following code will crash at `evpp/tcp_client.cc:26: evpp::TCPClient::~TCPClient(): Assertion `c->IsDisconnected()' failed.`
```C++
std::shared_ptr<evpp::TCPClient> client;
client->Connect();
... After client has connected.
client->Disconnect();
client.reset();
```