Seperate offers from libp2p

[binarybaron]

Currently the Offer returned by Alice to Bob is bound to the network channels. This introduces two major issues:

1. It's not really designed to be able to "just request an offer" without taking the other party up on it. If Bob does so, Alice sees it as "Bob closed the channel unexpectedly"
2. The offer duration (i.e. how long is the offer valid for) is integrated into the timeout of the libp2p channel. This is not ideal.

It'd be better to:

1. Have a protocol that can be called to request an offer. Alice returns a struct that contains the price, minimum amount, maximum amount, id of the offer and for how long the offer is valid (either relative or absolute time). This protocol can then also be used for the when list-sellers is called (where we don't intend to take anyone up on the offer directly)
2. Have another protocol for Bob to actually take Alice upon on her offer.