Extend Proof of Work requiring clients to perform a proof of work on session renewal.

[evilaliv3]

Proposal

I would like to propose to extend the proof of work mechanism requiring clients to complete a challenge to perform the session renewal.

The idea is to require the client to perform a dynamic proof of work mechanism varying from complexity LOW to HIGH based on a threshold.

Extending the thresholds defined in #4118 and adding thresholds_proof_of_work_operations_per_session = 50 we could for example require a client to renew the session every 60 seconds by continue to solve a proof of work. The proof of work complexity could be dynamically changed from a level LOW to HIGH based on the number of requests that the client is performing.

Motivation and context

Ticket strictly related to #4118 with same motivations.

[evilaliv3]

Closing the ticket since current version (5.0.12) implement the proof of work at every session renewal.

The complexity for the moment is kept constant.