[new feature] persistent swap listener

[dekdynamics]

Hi,

This is a feature request for implementing a persistent swap listener, similar to how trade-secret can list multiple trades.

Maybe like this:

bos swap
> Listen for swap request
> Number of available swaps: 5
> Direction:
 > I want to receive on-chain funds (or)
 > I want to receive off-chain funds
...

Offer parameters like ppm cost, or "track chain fees" could be useful. ie, maybe I only want to perform swaps under a cetain next-block fee estimate.

In addition it would be cool to use a bos advertise to alert your direct peers that swaps are available.