Fix #291

During an offense, the validator ID/ account ID is used to identify the faulty validator. It is then converted into an identity using identity_index_of from the identity pallet. This identity is subsequently used for all further checks, such as going online or disconnecting the validator. However, the offense system must be able to resolve any validator ID to an identity when processing past offenses that were reported. Therefore, the key should not be changed during this period, which corresponds to ReportLongevity, the maximum reporting duration.

The new behavior is as follows:

• An online smith cannot change its key.
• An offline smith cannot change its key if it is within the bond period, ensuring it can still be punished for past actions.
• An offline smith can change its key at any moment between the end of the last bond period and the go_online call. Even if the key is changed between set_key and go_online, the change will be reflected in the authority members' member data.