Support a session manager to manage connection with same SPACE

[horoc]

In java client, user can use a session manager to specify SPACE when getting a session, so why go-client doesn't support that feature, since it's maybe a common requirement of client.

SessionsManagerConfig config = new SessionsManagerConfig();
NebulaPoolConfig poolConfig = new NebulaPoolConfig();
poolConfig.setMaxConnSize(4);
config.setAddresses(Collections.singletonList(
    new HostAddress("127.0.0.1", 9670)))
    .setUserName("root")
    .setPassword("nebula")
    .setSpaceName("space")
    .setPoolConfig(poolConfig);
SessionsManager sessionsManager = new SessionsManager(config);
// Gets the session of the specified space
SessionWrapper session = sessionsManager.getSessionWrapper();
ResultSet resultSet = session.execute("......");