This time will make it possible to implement cooldown features on conflicting proofs.
This diff only adds the container and a method to update the time for a given proofid, which will be called in 2 places to set the cooldown:

• during a proof registration, to limit the conflicting proof rate
• after a vote is finalized, to limit the peer replacement rate upon acceptation by the network.

Note that this time uses std::chrono::seconds and not TimePoint so it can be mocked in the tests.

Ref T1854.