extract persistence event handling into standalone functions

[mattsse]

for readability it would be nice to have this code in standalone functions

https://github.com/paradigmxyz/reth/blob/c4aae6db361d2ac33fed18f25bb50158907723ee/crates/engine/tree/src/persistence.rs#L63-L73

like on_remove_blocks, on_save_blocks etc.