Make arbitrate deal able to slash miners for never posting the sector

if a miner and a client make a deal, and the miner never posts the sector, the client should have some recourse. The miner can never get paid for this data, but the client is out their fees for setting up the payment channel, their time, and bandwidth. So that kinda sucks for them.

This isnt easy, as we have to balance this solution against the client not being allowed to slash the miner twice for the same failed deal, all while avoiding putting information about each deal on chain.

No files, tests, or entry points are named. Start by examining how deal arbitration, sector posting, payment channels, and existing slashing are specified; done means defining a recourse mechanism that avoids double slashing and does not put every deal on chain.

Written by the indexing model from the issue text.