Support multi-hash in multi-trie via PlainDB (#1106)

* Temporarily pin trie to #2

* Use generic and delay trait object casting

Rust does not support super-trait upcasting

* Add PlainDB impl for Ephemeral

* Add PlainDB trait alias for completeness

* Use PlainDB for test TrieBackendStorage fetch

We always check overlay first for a storage fetch, which already checked null data. Using PlainDB here makes it work
nicer with other PlainDB overlays.

* Update trie reference

* Use HashDBRef in places when approriate

* Use PlainDBRef in places when approriate

* Update trie crate reference

* Remove unused HashDB::keys

* Patch dependencies

* Fix cargolock

* Update cargo lock again