Provide HTTP header Last-Modified on important URL's : /ldp , /display

[jmvanel]

These HTTP headers are the ones checked in other servers:
Last-Modified and / or Etags .

So, this is just coherent to provide, for example in the context of several SF instances mirroring each other.
The information for Last-Modified is currently displayed for each user user-modified triples; the more recent should be returned.