wsgetpeople cache / stats

[multiscan]

The fact of using api.epfl.ch instead of accessing directly the database makes wsgetpeople much slower than the original version. We have to do several api requests per person listed and, for long lists, this takes a lot of time. All api requests are cached for 24h. This helps but I think there should prefetch and cache all the requests to wsgetpeople in periods of low traffic.
The idea is to create a model store in the work database where we log the various requests, the frequency we get them, the last time we refreshed the cache etc. This will enable a possibly more efficient pre-caching mechanism.