[feature]: Do not keep MC data for blinded hops.

[ziggie1984]

Due to the ephermal nature of the blinded route and its hop pubkeys it does not make sense to persist this data. Therefore we should only keep this data in memory during the payment process and remove it afterwards.

See also #9316

[ziggie1984]

A very nice design approach was mentioned by @ellemouton:

We should use the new namespace feature of the mission-control system to save the blinded path mission control data there separately. When looking for a path we would have to merge those different namespaces together (which is the difficult part of this change) so we have the whole data available for pathfinding. Then during startup because Route-Blinding data is ephemeral we wipe this blinded route namespace.