Simplify Retrieval Provider when go-data-transfer revalidator refactor ships

[hannahhoward]

What

When filecoin-project/go-data-transfer#308 finishes, integrate it to make the retrieval provider much simpler.

Steps

• Drop tracking of data transfer statistics inside of retrieval provider
• Drop provider revalidator and migrate into expended regular validator, returning appropriate data limits
• Watch for pauses and send a voucher result programmatically with information about payment (for backwards compatibilty)
• Watch for finished transfers that enter finalization and send voucher result about final payments
• Move restarts into the revalidate function