Prevent the possibility of changing the threshold during Key Refresh #57
Description
After looking into the security update of https://github.com/ZcashFoundation/frost/releases/tag/frost-core%2Fv2.2.0, we decided that we should ourselves also do this check to prevent in the future from running refresh while dropping the threshold.
Currently this seems difficult to do as the threshold is not hardcoded in the implementation, nor in the key shares making the check impossible to do as a library user could always make the mistake of refreshing with different threshold than the old one.
The equality of threshold and old_threshold obscures an inconsistency in the lower
layers of the protocol as currently written.
Could be tied to #18
Acceptance Criteria:
Short term, document that threshold congruence is deliberate and that removing this
property could have security consequences, so that it is not accidentally changed without
the developer being aware.
Long term, update the APIs to not tolerate different threshold values during key refresh if
they are not intended to be changed.