These changes are primarily for Consul's UI, where we want to be more
specific about the state a peering is in.
- The "initial" state was renamed to pending, and no longer applies to
peerings being established from a peering token.
- Upon request to establish a peering from a peering token, peerings
will be set as "establishing". This will help distinguish between the
two roles: the cluster that generates the peering token and the
cluster that establishes the peering.
- When marked for deletion, peering state will be set to "deleting".
This way the UI determines the deletion via the state rather than the
"DeletedAt" field.
Co-authored-by: freddygv <freddy@hashicorp.com>
