multi: add sql backend support

[ziggie1984]

Still draft needs a bit more cleanup, so work in progress.

Fixes: #53

[ziggie1984]

example commands:

chantools walletinfo -r --db.backend sqlite --withrootkey --walletdb ./chain.sqlite

chantools walletinfo -r --db.backend postgres --db.postgres.dsn "postgres://lnd_migration:lnd_migration@localhost:5432/lnd_migration_db?sslmode=disable" --withrootkey

chantools -r --db.backend sqlite --db.sqlite.data-dir /regtest/data dropchannelgraph --single_channel 930186837164032 --node_identity_key 0356b85c9fd3d8da2e74e646e9f5b327d9b9c8538f0a817dcf72662fcad0cae49d