handle retrieval deals with zero price per byte

[dirkmc]

Prevent a crash when retrieval deal price per byte is zero.

Addresses filecoin-project/lotus#5140 filecoin-project/lotus#5409 (comment)

Improvements

Currently the flow is

• client sets up payment channel
• client sends a payment channel voucher
• provider sends some data
• client sends a payment channel voucher
• provider sends some data
• ...
• provider finishes sending data and requests final voucher
• client sends voucher
• provider sends "complete"
• client marks channel as complete

This PR handles the case that the retrieval deal price per byte is zero:
When the data has completed transferring

• provider doesn't request a final voucher
• provider immediately sends "complete"
• client waits for final data to arrive
• client marks channel as complete

Out of scope

In a future PR we should make more comprehensive changes for free retrievals:

• Avoid creating/funding a payment channel entirely
• Skip all the pause resume logic