Contract Pruning

[peterjan]

This PR adds contract pruning to the contractor. I'm following it up with a metrics PR. I've been testing it quite extensively on a separate (integrity) node and haven't ran into any issues. I even discovered a nasty race in the upload manager wrt uploading sectors. I chose to extend the upload manager with persistence, to avoid having the caller worry about "finishing" the upload after persisting whatever needs persisting in the bus. It passed 1200 iterations of TestSectorPruning so should be good.

[peterjan]

testing on areq - will merge tomorrow morning if things did not explode

[peterjan]

@alexfreska this PR adds a new field to the ContractsConfig called "Prune".

When set to true, the autopilot will try and prune deleted sector roots from contracts, one contract at a time, for a max duration of 10 minutes per contract. For old hosts this process takes quite a while, for new hosts it's fast and will make it so deleted data is also effectively deleted from the contract, making sure renters don't keep paying for storage they are not using.

Eventually this can/should be tied in with prune metrics, and perhaps also expose /api/bus/contracts/prunable data in the UI (e.g. together with the object stats, alongside reclaimable storage maybe). For now though, could we add this to the autopilot configuration and default it to false?