Handle chunks that cover multiple channels or times (Data Manager 5/9)

[frasercl]

This PR builds in the assumption that DataManager chunks cover exactly one channel and time point. This leads to a few modifications:

• ChunkSources must provide a mapping from chunk IDs to "storage keys," where each storage key may cover more than one channel or time point
• Chunk requests must be tracked separately from individual chunks, since they might not correspond 1-1, and requests may only be cancelled if every chunk in the request has been cancelled.

Also introduces the SlotMap data structure for a little help tracking requests efficiently.