Who controls the "Payment Hub" in tumblebit?
Is it another protocol or company or is there something else / someone else involved?
The notion is that anyone with sufficient Bitcoins to act as collateral for the payment channels could setup a TumbleBit payment hub. This would likely be a company.
The more users a TumbleBit payment hub has the greater the privacy and utility offered so one would expect a small number of payment hubs with many users.