end of segments reached message is shown erroneously

Using the next / previous segment button in a task with many readonly segments can lead to wrong "end of segments reached" messages. This happens because on using the next / prev button only the actually buffered segments in the client are considered.
If the current buffer contains only readonly segments after the current segment, no next one is found. This triggers then the message.
Solution: we deliver from backend the id of the first and last segment of the current filtered list.