* 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