journaling with efficient versioning

2.11: adde/journaling/efficient versioning:

. notice reverse diff can't be done by journaling,
needs exact replace commands:
eg, suppose block has x's
and I use command to replace all x's with y's,
but there are already y's .
then a reverse journal changing y's back to x's
is going to change too many y's
and therefore won't represent the previous version .
. reversing must keep track of either whole block versions
or use address of changes
-- in some cases the instruction for reverse
would be heavier than just keeping both versions .