state token for library requests #93
Description
A library response returns a 13-digit numeric state-token in the response headers.
Using this state token in the url query on the next request like /1.0/library?state_token=1234567654321 will return only changes since the state take was given.
If there are no new content, the json response body items list is empty, the total-count header entry is 0 and the state token is in the header is unchanged. If there are changes in the library, a new state token is obtained, which should be used next time.