Ticket 47582 - agmt_count in Replica could become (PRUint64)-1
    
    Bug Description:  agmt_count can go negative, and lead to a crash.  This can
                      happen when creating an agreement fails, and we lower the count
                      before it was actually incremented.
    
    Fix Description:  Move the agmt incrementing to agmt_new_from_entry(), as
                      this same function can also cause the count to be decremented
                      if something fails.
    
    https://fedorahosted.org/389/ticket/47582
    
    Reviewed by: richm(Thanks!)