- DONE: write FragmentIndex
 - DONE: adapt SortKey ... naming! :
    - FragmentIdIndex (index as in db index)
        - compare(idA, idB)
    - SortKey
        - FragmentId
        - EventIndex
 - 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