Consider counting retrievals of format / spec / statistics documents

[bansp]

This may be a big absentee feature, in fact -- we could use this for more than one goal: seeing if we need to bother about breaking links (compare #479 ) but also simply to see if the SIS is useful and how useful it is / becomes.

[bansp]

Write an e-mail to Dieter to ask which system it's best to use, in order to conform to the other parts of the infrastructure.

[bansp]

Eliza mentions that requests alone could be logged at the server level.