In this commit, we add a new method that lets us insert a sidecar ticket
as well as a bid in an atomic manner. We'll use this later when offering
this new type of automated ticket.
Both the trader offering to buy a sidecar channel for another node as
well as the recipient node will need to store the involved sidecar
tickets in their local databases. We add a simple store interface and
its implementation that will be used in future commits.
