Client's NextDealID is persisted to disk

[hannahhoward]

As a retrieval client, I would like my next deal id to survive between restarts, so that my client never sends duplicate ids, and providers can treat it as a unique identifier for my machine

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In:

• Write next deal id to a data store
• Rewrite it each time a new deal is made
• Read it in when the retrieval client starts up

Acceptance criteria:

• create retrieval client with a datastore
• make a deal
• shut down client
• create a new cliient with the same datastore
• make a deal
• verify deal ID was incremented