Motivation
Currently, our Store contains an Arc<Mutex<dyn StoreEngine>>, but the mutex is not really needed for the libmbdx backend, it is only needed for the InMemory one. This means we should be able to move the mutex into the in memory engine and leave the libmdbx mutex-free.
This PR proposes moving this mutex into the InMemory engine and changing the structure of Store to:

pub struct Store {
    engine:  Arc<dyn StoreEngine>
}

Description

• Change Store.engine type from Arc<Mutex<dyn StoreEngine>> to E: StoreEngine,
• Wrap in memory store engine in Arc<Mutex<>>
• Rename implementations of StoreEngine to LibmdbxStoreEngine & InMemoryStoreEngine and expose them publicly
• Add method new_temp for Store to be used in tests instead of creating and removing directories "by hand".

Closes #issue_number