For ZBlk1 we already compare ZData content about whether it was changed
compared to data already stored to DB, and do not store it twice if data
is the same.

However ZBlk itself is always marked as changed, if corresponding memory
page was dirtied. This results in transactions like

Trans #33915309 tid=03b6944919befeee time=2016-04-17 22:01:06.034237 offset=140320105842
    status=' ' user='...' description='...'
  # ... other parts, but no ZData here
  data #2 oid=000000000026fc4c size=79 class=wendelin.bigfile.file_zodb.ZBlk1

where ZBlk1 is committed the same without necessity.

NOTE we cannot avoid committing ZBlk in all cases, because it is used to signal
    other DB clients that a ZBlk needs to be invalidated and this way associated
    fileh pages are invalidated too.

    This cannot work via ZData, because ZData don't have back-pointer to
    ZBlk1 or to corresponding zfile.