Limit disk read and data upload to latest sync

When reading files from disk, we now limit the files read to those that
might contain measurement data later than the latest synchronization
time.

When returning measurements from a file that we have read from disk, we
now limit the measurements to include only those that are later than the
latest synchronization time.

This means that we are now much more efficient when there is lots of
files on disk. It also means that we no longer should upload duplicate
data.