ext_storage_get_version_1 behavior w.r.t. child tries

[tomaka]

When reading the location of a child trie using ext_storage_get_version_1, the trie root hash of the child trie should be returned.

But there's a twist: the returned hash only takes into account the child trie writes that have been performed until the last time ext_storage_child_storage_root_version_1 was called.

On the other hand, ext_storage_child_storage_root_version_1 always recalculates the trie root hash of the child trie and takes into account all the writes that have been performed.

So for example if you call ext_storage_child_set_version_1, then ext_storage_child_storage_root_version_1, then ext_storage_child_set_version_1 again, then read the child trie root hash using ext_storage_get_version_1, it returns the hash that ext_storage_child_storage_root_version_1 returned.