deterministic indexer backup so that all nodes produce the same bit-by-bit snapshot

[altergui]

right now the content of the indexer db is deterministic, but the backup is not since it's in binary format. to make it deterministic we should instead do something equivalent of a "sqlite3 .dump"
i.e. that the backup is a (compressed) bunch of SQL INSERT (etc) statements instead of native binary sqlite3 format