Improve the bandwidth token payout mechanism

[devos50]

As we have seen in issue #4234, the current payout mechanism is functional (as most of the time, people are earning bandwidth tokens for running Tribler idle), but it can and should be improved. Note that #4248 and #4250 addressed issues during payout.

• Get rid of the 5-second window during which a payout should be made - after receiving a destroy message, a circuit continues to live for another 5 seconds. During this time, a payout should be made. If the payout takes longer, the payout will most likely not reach the exit node.
• Remove requirement for honest behaviour of intermediaries - the current mechanism heavily relies on intermediary (relay) nodes in a circuit acting honest and transferring tokens to the next hop in a circuit. This can easily be abused. Possible solutions: incremental payouts? Detect whether a node did not pay and not join their circuits?
• Improve the visualization of the trust score. Instead of calling it MBs given/taken (which is the technical perspective), we should name it investment/yield for instance.
• Long term: Implement deferred payouts - this prevents a correlation attack based on TrustChain records, since one can infer which nodes did payout each other.

As more issues are discovered, they will be documented here.

[devos50]

I documented the process of payouts here.