avoid aggressive socket connection error callback

[jayjlu]

Avoid aggressive socket connection error callback and some javadoc fix.

    public void onClose(ConnectorSocket socket, Throwable error) {
        runningSockets.remove(socket);
        idleSockets.remove(socket);
        if (error == null) {
            addAnyMissing();
        } else {
            // if error, adding back sockets with backoff
            addAnythingMissingWithBackoff(); 
        }
    }

The majority change is here. this onClose() method will be invoked for normal close and error close. we want to make sure it done the backoff when error happened (at least do the reconnect with backoff start from 502 ms till 10 second) .