A sidecar ticket will always use the minimum version needed to encode
all the relavant information for that offer. In that way the tickets
from a provider with a newer version can be redeemed by recipeients
using older versions as long as the ticket does not encode new
functionalities.
If we're the provider of a sidecar channel then we don't actually have
the private key of the funding multisig key. Therefore we cannot
re-derive it on demand and have to use the key provided in the sidecar
ticket instead.
To make iterating over batch IDs easier, we add a new helper function
that returns all batch IDs starting at the start key (inclusive) and
ending at the end key (inclusive) by decrementing the public keys. If
the end key cannot be reached by decrementing the start key by a maximum
of 10k times, we abort and return the first 10k items (without the end
key).
