Duniter authority members pallet

In a permissioned network, we have to define the set of authorities, and among these authorities, the ones validators in the next session. That's what authority members pallet does. In practice:

• it manages a Members set with some custom rules
• it implements the SessionManager trait from the session frame pallet

Entering the set of authorities

To become part of Duniter authorities, one has to complete these steps:

1. become member of the main web of trust
2. request membership to the smith sub wot
3. get enough certs to get smith membership
4. claim membership to the set of authorities

Then one can "go online" and "go offline" to enter or leave two sessions after.

Staying in the set of authorities

If a smith is offline more than MaxOfflineSessions, he leaves the set of authorities.

Some vocabulary

Smiths are people allowed to forge blocks, but in details this is:

• smith status required to become an authority
• authority status required to become validator
• validator status required to add blocks