- DONE: write FragmentIndex
 - adapt SortKey ... naming! :
    - FragmentIndex (index as in db index)
        - compare(fragmentKeyA, fragmentKeyB)
    - SortKey
        - FragmentKey
        - EventKey (we don't use id here because we already have event_id in the event)
 - write fragmentStore
 - adapt timelineStore
 - adapt persister
    - persist fragments in /sync
    - load n items before and after key
    - fill gaps / fragment filling
 - add live fragment id optimization if we haven't done so already
 - lets try to not have to have the fragmentindex in memory if the timeline isn't loaded