When importing an existing delegation, allow to only keep the delegation
levels up the given public key.
The session key is expected to match the last level of the delegation, so only this last level is checked which is good for performance reason.
This diff will make it possible to pass a delegation to a node (feature to be added) where the delegation contains the session pubkey but is not the last level.
This makes it possible to build upfront a single delegation for several nodes without having to do a 1:1 match.