Make child tries work properly (#743)

* Remove special child tries handling from host.rs

* Put back the exceptions to runtime_host.rs

* Reorganize a bit the trie root calculation code paths

* Recalculate stale child tries when calculating the main trie root

* Remove unused import

* Handle if child trie has been destroyed

* Simplify code given that child tries can't be marked as destroyed

* Remove TODO in host.rs

* PR link

* ExternalStorageRoot::resume no longer accepts an Option

* Update documentation

* More doc

* More doc

* Add TODO