Do not update statisitics if buffer has not changed

Do not update statisitics if buffer has not changed

Rely on the newly-introduced Buffer::id() to skip statistics
computation if the id is the same as last time. This will reduce the
annoyance of updates triggering at random times.

Take this occasion to clean code up:

- add 'skip' parameter (true by default) to Statistics::update to indicate
  that the insets that do not produce output should be skipped.

- use a trailing underscrore for private members