Due to the internal fold over buckets returning an un-reversed
accumulator, the API bucketlist code caller's fold fun traversed the
bucket list in reverse order. This lead to some inconsistencies when
comparing a buckelist of all buckets, vs, first bucket only. i.e. the
'first' bucket passed to the foldfun was in fact the last bucket read
from the ledger.
Previously there was no is_empty check, and there was a workaround using binary_bucketlist. But what if there were many buckets - this is a slow seek (using get next key over and over).
Instead have a proper is_empty check.
Initial commit to add head_only mode to leveled. This allows leveled to receive batches of object changes, but where those objects exist only in the Penciller's Ledger (once they have been persisted within the Ledger).
The aim is to reduce significantly the cost of compaction. Also, the objects ar enot directly accessible (they can only be accessed through folds). Again this makes life easier during merging in the LSM trees (as no bloom filters have to be created).
Initially with basic tests. If the SlotIndex has been cached, we can now use the slot index as it is based on the Segment hash algortihm.
This looks like it should lead to an order of magnitude improvement in querying for keys/clocks by segment ID.
This also required a slight tweak to the penciller keyfolder. It now caches the next answer from the SSTiter, rather than restart the iterator. When the IMMiter has many more entries than the SSTiter (as the sSTiter is being filtered but not the IMMiter) this could lead to lots of repeated folding.
Introduce a dedicated module for all the different fold types. Also simplify the list of folders by deprecating those folds that should eb achieveable by fold_heads/fold_objects type folds but with smarter functions.
Makes sure that the fold functiosn also have better spec coverage, and are dialyzer checked.
