* multi: extend InvoiceDB methods with a context argument
This commit adds a context to InvoiceDB's methods. Along this refactor
we also extend InvoiceRegistry methods with contexts where it makes
sense. This change is essential to be able to provide kvdb and sqldb
implementations for InvoiceDB.
* channeldb: restrict invoice tests to only use an InvoiceDB instance
* docs: update release notes for 0.18.0
Now that we have the new package `lnd/channeldb/models` we can invert the
depenency between `channeldb` and `invoices`.
- Move all the invoice related types and errors to the
`invoices` package.
- Ensure that all the packages dealing with invoices use the types and
interfaces defined in the `invoices` package.
- Implement the InvoiceDB interface (defined in `lnd/invoices`) in
channeldb.
- Add new mock for InterfaceDB.
- `InvoiceRegistery` tests are now in its own subpacakge (they need to
import both invoices & channeldb). This is temporary until we can
decouple them.
