Move to using flumedb

[nikolai-b]

Currently we store in json. Use flumedb and flumelog-offset.

Background 👀 to flumedb video

[ameba23]

This is now implemented, in that we no longer use a local file, and get all ssb-sourced information through our ssb plugin.

But electrum still writes to a local file internally, and when we stop using electrum we need to think about how we will store the equivalent information. It would be great to use some simple key-value store, but it needs to be encrypted. We need to look into whether is possible to write to our local flume db without publishing to ssb if that makes sense. and think about security - privates keys will need to be stored, which under no circumstances should be published to ssb, regardless of whether they are encrypted.