Using many extra resources may cause high load at proxy server, because
the process needs to get all of the requested objects concurrently.
This change introduces a new option to limit maximum number of extra
resources per request to avoid DoS attack by too many extra resources.
If a request contains extra resources over the limit, then the request
is rejected at an early stage. The default value is -1 which means
unlimited. In case this option is set to 0 then users are not allowed
to use extra resources at all.
This also refactors handling of extra-resources header. One side
benefit of the refactoring is that now users can use additional inputs
when executing storlet over PUT requests.
Change-Id: I0ea7d78614f2b1ef5bf4961d2d5fe773264ef448
8401b6c581