Revert "Save session only when there is change since the last serialization"

This reverts commit 1944d8db58555c08b5f069a2e987fcfdd5aad997.
Save session only when there is change since the last serialization

This is a slight optimization not to cause useless disk access.  This
only applies to saving session automatically (see
--save-session-interval).  aria2.saveSession and serialization at the
end of the session are always performed as before.

When serialization, we first check that whether there is any change
since the last serialization.  To do this, we first calculate hash
value of serialized content without writing into file.  Then compare
this value to the value of last serialization.  If they do not match,
perform serialization.