bigfile/zodb: Reuse __setstate__() in ZBlk.__init__()

- we have logic to init ._v_zfile and ._v_blk there

- the same for ._v_blkdata - logic to init it is there + it is better to
  set variables right from instance creation, not hoping "it will be set
  outside from master"

  NOTE ._v_blkdata = None means the block was not yet loaded and
  generally fits into logic how ZBlk operates and thus the change is ok.

bigfile/file_zodb.py

@@ -118,9 +118,7 @@ class ZBlk(Persistent):
 
     # ZBlk as initially created (empty placeholder)
     def __init__(self):
-        self._v_zfile   = None
-        self._v_blk     = None
-        # NOTE ._v_blkdata is not set - the master will set it from outside
+        self.__setstate__(None)
 
 
     # make this ZBlk know it represents zfile[blk]