ZBlk* objects are intermediate ZODB object in between data stored in
ZODB and memory pages managed by virtmem. As such, after they do their
job to either load data from DB to memory, or store from memory to DB,
it is not needed to keep them alive with duplicate content thus only
wasting memory.

ZBlk0 cares about this detail via "deactivating" ._v_blkdata in
loadblkdata() and __getstate__() prologues.

ZBlk1 did the same for load path in loadblkdata() prologue, but for
.__getstate__() it was not directly possible, because for ZBlk1 the
state is IOBTree, not one non-persistent object, and thus it first needs
to be processed by ZODB together with its subobjects on its way to
storage and only then all they deactivated.

So 13c0c17c (bigfile/zodb: Format #1 which is optimized for small
changes) only put TODO for memory-page -> DB path about not wasting
memory this way.

But the problem is relatively easy to solve:

    - we can deactivate ZData objects (leaf objects in ZBlk1.chunktab
      btree) by hooking into ZData.__getstate__() prologue;

    - we also need to care to deactivate chunks right away, which
      setblkdata() loaded to compare .data and found them to be not
      changed

This way we do not waste memory keeping intermediate ZData objects alive
with the same content as memory page after commit.

/cc @Tyagov