pending optimizations on Arbo

[p4u]

During the Arbo refactor performed on this PR #1080 I realized of two potential optimizations that could be made to increase speed on Add/Delete and Update methods.

1. Instead of removing the intermediate orphan nodes SYNC (after the up() operation), it could be done ASYNC. The only thing we need to care about is to find a mechanism that can recover the list of nodes to remove after a program forced stop.

2. When deleting recursively calling deleteWithTx() the orphan nodes (to later on add them again in the correct place). Instead of starting from the current Root we could execute the delete on a subtree, where the root is the current neighbour node.