Adds a function to collect all the values with a key larger or equal.
Adds some shortcut function to get the indexed values.
Fixes some inconsistencies in the protocol examples.
Adds a version log data structure.

It maintains an history of versions and can provide a cummulative
patch to get from any known version to the last one.
Moves items outside, removes barrier's keys, integrate the version log.

* The value map has been moved outside the page so it can be shared
  between multiple views.
* The barrier now only uses the smallest and largest value, the keys
  have been removed.
* The page now uses the erod_document_verlog data structure to store
  the patches and manage the page version.