Delete Memberships and replace functionality. Add some methods to
`Membership` trait to deal w/ collapsing into one type both kinds of
memberships (stake and DA).

  * avoid passing membership into `is_valid_cert
  * for DA, avoid proxying threshold through `Threshold` trait
  * remove `Topic` param from `Membership::new
  * Split cert impls by marker (#3891)
  * add membership methods to Cert trait
  * remove non-existent tests from justfile

We can keep the old name where we only have one membership type to
keep the diff smaller.

Always use quorum for leader selection