`MemoryStore` allows duplicate `ChunkIdentifier`'s

MemoryStore currently relies on RelationalLinkedChunk::apply_updates to manage Update's to its internal data. This function, in turn, relies on RelationalLinkedChunk::insert_chunk when adding new chunks, which does not check whether the given ChunkIdentifier exists already.

To confirm this behavior, one can add the following integration test to EventCacheStoreIntegrationTests.

impl EventCacheStoreIntegrationTests for DynEventCacheStore {
    async fn test_linked_chunk_identifiers_are_unique(&self) {
        self.handle_linked_chunk_updates(
            LinkedChunkId::Room(room_id!("!r0:matrix.org")),
            vec![
                Update::NewItemsChunk { previous: None, new: ChunkIdentifier::new(0), next: None },
                Update::NewItemsChunk { previous: None, new: ChunkIdentifier::new(0), next: None },
            ],
        )
        .await
        .unwrap_err();
    }

    // -- snip --
}

My guess is that this behavior is not desirable. It also happens to be inconsistent with other implementations of EventCacheStore.

Start with RelationalLinkedChunk::apply_updates and insert_chunk in crates/matrix-sdk-common/src/linked_chunk/relational.rs, then review EventCacheStoreIntegrationTests and the other EventCacheStore implementations. Run the proposed uniqueness integration test; done means duplicate ChunkIdentifier updates are rejected consistently with those implementations.