[Chainstate] Add support for an ephemeral MARF

[jcnelson]

In order to fully implement #6346, we need a ClarityBackingStore implementation with an ephemeral MARF. It can accept reads and writes, but the writes will never hit the disk and will be discarded once the implementation is dropped. This can be achieved without touching the MARF code proper; instead, we should create a wrapper around the MARF similar to ReadOnlyMarfStore<'_> and WritableMarfStore<'_>.