[Epic] channeldb+invoices: create native SQL schema for invoice storage

[Roasbeef]

We recently added postgres support, but in a way that just maps things to a naive key-value table. In order to take advantage of increased queryability, and scalability gains, we should convert the invoices (which can be heavily loaded on large nodes that help to operate services) to a native SQL schema. Along the way, we'll want to abstract away the invoice registry's storage behind an interface, so this new data format can be swapped in at runtime, or during initial configuration of the daemon.

Steps to Completion

• Eliminate the invoice registry's dependency on the raw channel db pointer: https://github.com/lightningnetwork/lnd/blob/master/invoices/invoiceregistry.go#L106. This can be done by introducing a new slim interface that only requires the invoice related methods we use. #6842
• Expose the new interface as part of the set of defined DatabaseInstances. This'll allow projects that "wrap" lnd to pass in their own custom implementations, which may use a different DB backend all together. #6954
• Create a new SQL-native invoice schema.
  • Tools like sqlc look interesting here as it lets you mainly write SQL in a single file, and have the CRUD code be generated. We use this in some backend products at Lightning Labs now.
  • Alternatively, since sqlc doesn't support sqlite fully (though I think if you don't use any sqlite specific features it works), we could look into using other ORMs.
• Implement a migration tool to be run automatically given a relevant set of flags, or manually via a migrate-then-shutdown flag/script.

Related to:

• add SQLite3 backend option #6176

• Create a new SQL-native invoice schema:
  • sqldb: new package supporting sql backends #7343
  • sqldb: add invoice schema and sql queries #7354
    * [WIP] sqldb: InvoiceDB interface implementation #7357
  • sqldb: InvoiceDB implementation #8052

[bitromortac]

Started to look into the first two points as of assignment to #6954.