hummingbird: bidirectional reservations

[juagargi]

Add support for bidirectional reservations.
A bidirectional reservation doesn't exist in Hummingbird. The reservations in Hummingbird are unidirectional.
This PR adds the ability of sending a preexisting Hummingbird reservation to the destination,
so that the destination can unpack it and use it as a path in the reply packets.

The typical scenario for this feature is communication between a client and a server.
The client would request a regular, forward reservation from the client to the server.
Additionally, the client could also request the reverse reservation, i.e. from the server to the client,
so that the server would use it to reply to the client.

The reverse reservation information is carried inside an end to end extension header,
typically only once (as the packets are QoS protected by the forward reservation).
The server just needs to unpack this information and recreate the reverse reservation.
The server needs to keep in memory this reservation, at least for its valid lifetime,
or until a new reverse reservation is sent by the client.

The setup of a bidirectional reservation by the client is easy.
An extra parameter of reverse bandwidth has been added to the OneShotReservation
high level API.
The server just needs to use the HummReplyPather as a reply pather.

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This change is 

[juagargi]

Reviewed.