Update src/php/whatsprot.class.php

This is my first time to contribute a repository. Maybe my code is not very good.
I suggest that the msgid should be calculated by the class in the form of non-repeated
'timestamp-msgCounter'.
Also, I think it is better to send messages without waiting the response from server about the last message sent.
Moreover, I implemented an array called _sentQueue to store the sent message node with msgid and remove the corresponding element in the array if the server confirmed the received message
If this is accepted, a function should be implemented later to deal with the sent message but the server does not confirm that the message is received. (That is, the messages stored in _sentQueue)