Give every object in solitude an e-tag (bug 895172)

Still missing:

* [x] a validation about the `epoch_micro` approach
* [x] SQL queries to create the added field
* [x] a discussion about the situation of PUT/PATCH requests without an `etag`
* [x] a factorization of the code to extract mixins/resources dedicated to a given REST framework (note that `UpdateModelMixin` is not in use/tested for now)
* [x] switching to `counter` + `pk` as a unique etag