Remove token after used or when expired

This commit will close issue #96 

1) Remove token after first use
2) Remove token not used after first use > 24h
3) Semi rollback of previous commit: we must check matricola and token when signup/reset so they have to be in knownKeys. Instead the removal of hash was correct
4) Some mini debug text improvement

Tested on live Highgarden

To use backend.writeStudentInfo() we have to pass matricola.