[C++] Fix incorrect connect timeout implementation

[BewareMyPower]

Fixes #10721

Motivation

Currently the connection timer stops after the TCP connection is established immediately (the state becomes TcpConnected). However, the connect phase should also include sending the CommandConnect request and receiving the CommandConnected response from broker successfully. For example, in a case like #10721 described, if the Pulsar broker received a SIGSTOP signal and became pending, the TCP connection can be established but no response could be received from a stopped broker. Then the client would hang forever.

Modifications

Stop the connection timer only after the ClientConnection's state becomes Ready, which means the client has received the CommandConnected response successfully. It's also consistent with Java client's implementation, see

pulsar/pulsar-client/src/main/java/org/apache/pulsar/client/impl/ClientCnx.java

Lines 329 to 330 in 235e678

	connectionFuture.complete(null);
	state = State.Ready;

Verifying this change

• Make sure that the change passes the CI checks.

It's hard to simulate the scenario in unit test. But we can simply reproduce it in local env.

1. Run a pulsar standalone and send SIGSTOP signal by pkill -SIGSTOP -f pulsar.
2. Run any C++/Python client with this fix to connect to the standalone.

>>> import pulsar
>>> c = pulsar.Client('pulsar://localhost:6650')
>>> c.create_producer('xxx')
2021-09-02 11:54:38.828 INFO  [0x10d72ae00] ClientConnection:181 | [<none> -> pulsar://localhost:6650] Create ClientConnection, timeout=10000
2021-09-02 11:54:38.828 INFO  [0x10d72ae00] ConnectionPool:96 | Created connection for pulsar://localhost:6650
2021-09-02 11:54:38.969 INFO  [0x700003136000] ClientConnection:367 | [127.0.0.1:62784 -> 127.0.0.1:6650] Connected to broker
2021-09-02 11:54:48.973 ERROR [0x700003136000] ClientConnection:532 | [127.0.0.1:62784 -> 127.0.0.1:6650] Connection was not established in 10000 ms, close the socket
2021-09-02 11:54:48.973 ERROR [0x700003136000] ClientConnection:572 | [127.0.0.1:62784 -> 127.0.0.1:6650] Read failed: Operation canceled
2021-09-02 11:54:48.973 INFO  [0x700003136000] ClientConnection:1495 | [127.0.0.1:62784 -> 127.0.0.1:6650] Connection closed
2021-09-02 11:54:48.973 ERROR [0x700003136000] ClientImpl:188 | Error Checking/Getting Partition Metadata while creating producer on persistent://public/default/xxx -- ConnectError
Traceback (most recent call last):
    # ...
_pulsar.ConnectError: Pulsar error: ConnectError

we can see after 10 seconds (the default connect timeout), it failed.