Ticket #2750: fixes to HTTP caching behavior across login/logout boundaries

* now ignoring if-modified-since if we failed an etag if-none-match comparison, per spec
* now including a hash of user id/nickname in most etags, so we'll update the view properly after login/logout

For API methods, checking the API-auth'ed user. (Many change results to include things like 'you're subscribed to this user' or 'this is one of your favorites', so user info is again needed)

There'll still be some last-modified stamps that aren't including user info properly, probably.

plugins/Autocomplete/autocomplete.php

@@ -79,6 +79,7 @@ class AutocompleteAction extends Action
     function etag()
     {
         return '"' . implode(':', array($this->arg('action'),
+            common_user_cache_hash(),
             crc32($this->arg('q')), //the actual string can have funny characters in we don't want showing up in the etag
             $this->arg('limit'),
             $this->lastModified())) . '"';