move RepairThreshold to renter package and fix related off-by-one errors

I noticed a few spots where we weren't treating the RemoteRepairDownloadThreshold constant correctly. Also, we no longer use it as a remote repair threshold, we use it as a general repair threshold. And also, it belongs in the renter package, which makes decisions around repairs, not in the siafile package.

I moved the constant to the correct place, cleaned up some related constants, and stripped out some code that was download specific since it's no longer necessary.