Here we add an explicit InboundFee field to the ChannelEdgePolicy
struct. Then, in the graph KVStore, at deserialisation time, we extract
the InboundFee from the ExtraOpaqueData. Currently we do this at higher
levels but we are going to move it to the DB layer so that when we add
the SQL implementation of the graph store, we can have explicit columns
for inbound fees. We need to account for the fact that we may have
invalid TLV already persisted though and we dont want to fail if we
deserialise those necessarily. So we return ErrParsingExtraTLVBytes now
if we fail to parse the extra bytes as TLV and then we let the callers
handle it similarly to how ErrParsingExtraTLVBytes is handled in that we
dont necessarily fail if we receive one of these errors.
As of this commit, we can now expect the InboundFee field of a
ChannelEdgePolicy to be set (if inbound fees are set on the policy) for
any update that we read from disk.
All the structs defined in the `channeldb/models` package are graph
related. So once we move all the graph CRUD code to the graph package,
it makes sense to have the schema structs there too. So this just moves
the `models` package over to `graph/db/models`.
