Membership trait changes to support PoS

[pls148]

Reference meeting notes

• HotShot clones the Membership object when receiving it, wrapping it in Arc<RwLock<>>
• Remove requirement for Membership to implement Clone
• Add sync_l1 and add_epoch_root functions, which return an Option<FnOnce(&mut Membership)> or similar. Functions should be called with a read lock held, then upgrade to write lock if a callback is returned. Default implementations should return None. A static helper function can be used to indicate if each function should be called at all.
• Convert the existing Membership methods to async
• Audit existing code to avoid Membership calls while holding other locks