MPC-Contract V2 (key event coordination + multi domain)

[kevindeforth]

Follow-up to #253 and #266.

To be done before the audit:

• [mpc-contract] Improve signature state #241
• KeyEvent apis should not return Error #261
• Adapt high-level unittests to new contract logic. #186
• integrate contract v2 with mpc node #262
• update readme and documentation
• Contract V2: vote_new_parameters should take an epoch ID #276
• start_keygen_instance/start_reshare_instance should take a KeyEventId #277
• Test contract V2 migration logic #280
• leader should vote abort in case the key event was started without them #297
• increment version of contract and node?

Can wait after audit:

• Add contract tests to CI #279
• Add more tests for ThresholdParameters::validate_incoming_proposal #289
• contract must enforce a single domain for ecdsa signature scheme #298
• measure gas usage (pytests show nothing concerning, so this can be postponed)