Make Chantools useable with LNDs new native SQL schema

Chantools currently assumes LND stores its data in bbolt key-value files (channel.db, graph.db, wallet.db, macaroons.db, etc.). LND is moving to a native SQL backend (SQLite/Postgres) where channel, graph, invoice and payment data live in relational tables rather than in the kvdb bucket structure. On a node running native SQL there is no channel.db file for chantools to open, so all commands that operate on the channel database currently break for those users.

We should make chantools able to read from (and where applicable write to) LND's native SQL databases.

Start by tracing the commands that open channel.db and the related graph.db, wallet.db, and macaroons.db files, then compare their assumptions with LND's native SQLite/Postgres schema. Done means chantools' channel-database commands work on native SQL nodes, including writes where supported, without requiring channel.db.

Written by the indexing model from the issue text.