Need to change the client library for reciept modacks using the following logic:

1. Client library receives a message from the service on streaming pull (already happens)
2. Client library issues the first modack (aka receipt modack) as soon as a message is received (already happens)
3. Client library waits for this modack (receipt modack) to succeed before sending the message to the user via callback.
   This is new
   Until now, IIUC, the modack will be executed asynchronously and the message will be sent to the user via callback.
   If the modack fails with temporary error, retry it
   If the modack fails with a permanent error, don't log it and move on.
4. If the message is sent to the user via callback, client library extends the lease periodically by executing modack calls (already happens, no change needed)