server-connection: Correct timestamp handling (based only on existing insns)

No need to store a server timestamp, and it was buggy anyhow. Now we
just read all insns we have and send the highest numbered one.
robot-framework: Correct client-already-has-server-data test

43be2dcaaeea8c86d73706ce7ab38b0ebb11ba25

broke this test since remote/ServerTimestamps.xml is no longer used.