First implementation:  mmap a file

What goes wrong?
Atomicity:
What if server crashes between A -= $1 and B += $1?
What if client crashes/cancels between two operations?
Consistency:  read-modify writes interfere
           R = B.balance;
                                R = B.balance;
                                R = R + $1;
                                B.balance = R;
           R = R + $1;
           B.balance = R;