Implement Database::with

[piperswe]

• Allow creating an ephemeral database value with a transaction's results in-memory, and everything else backed by the main connection's storage backend.
• See https://docs.datomic.com/on-prem/clojure/index.html#datomic.api/with
• Depends on Add an in-memory storage backend #34
• Depends on Add an in-memory cache on top of the storage backend abstraction #35