Generalize the codebase to run in accountable and non-accountable modes

[sr-gi]

Currently, the tower runs only in accountable mode, meaning that signatures of agreement are sent by the tower to the users so it can be held accountable in case of misbehaviour.

While this may make sense for towers run by third parties, it may not if the same person is running the tower and the client. Running in accountable mode means having to store the signed receipts by the client, which may not be desirable in this case.

Generalize the tower so it can be run in both modes. This should be configurable via compilation flags so the tower is run in one of the two modes (and advertised to the users during registration).

Further info: https://www.summerofbitcoin.org/project-ideas-details?recordId=recYp3MeVaI79Jocg

DISCLAIMER

This is a Summer Of Bitcoin (SOB) project. If you're considering tackling this but you are not part of SOB, please leave it until SOB is over. After that, feel free to jump in.