[activexml] cache all HTTP responses

That is all responses that have an etag. For those we will query the
backend/api only for changed answers. Of course the backend does not
support etags (yet) and the api will generate the etag according to
the generated content, so this is just a first step to perfect caching
[api,webui] use dalli instead of memcache-client
[api] check etag in one test route: /configuration
another test
[api] initial porting to rails 4