This really just required care around calculating the expected B-tree id
and rbyd id (which are different!).
B-tree append, aka B-tree push with id=weight, is actually the outlier.
We need a B-tree id that can identify the rbyd we're appending to, but
this id itself doesn't exist in the tree yet, which can be a bit tricky.
This involves many, many hacks, but is enough to test the concept
and start looking at how it interacts with different block sizes.
Note only append (lfsr_btree_push on the end) is implemented, and it
makes some assumption about how the ids can interact when splitting
rbyds.
Christopher Haster