Delete from the BPT [#3338]
Closes #3338 (closed). Adds a delete function to the BPT. Removes the tree merge logic - instead the update list is retained and committing a BPT in a nested batch always pushes the update list instead of maybe merging trees. So in a BPT in a nested batch the only thing the tree is used for is calculating the new root hash, if requested.
Changelog: feature