Tree hook for persisting blocks

[Rjected]

In the new engine design (#8742) we have a persistence thread, which can persist blocks that are in-memory. The in-memory tree actually contains the blocks, and should be responsible for telling the persistence task to write some of those blocks.

A PersistenceHandle should be added to the new tree, and the tree should send an action when the current canonical chain exceeds a certain number of blocks. This will mean collecting and cloning a section of the tree, creating an action, and sending it using the handle. This will also require implementing a hook that determines when to do this, based on the length of the canonical chain.