State machine local child root cache. (#9107)

* cache root for child api.

* minimal testing

* Reset cache on test 'set_root'.

* Update primitives/state-machine/src/trie_backend_essence.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update primitives/state-machine/src/trie_backend_essence.rs

* Update primitives/state-machine/src/trie_backend_essence.rs

* Renaming to 'reset_cache'.

* correct rust fmt

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>