WIP: migrations

[jsparber]

the order off operations for the migrations would be:

• read the operations from the old DB, since operations are using only a hash of the logId instead of the logid directly we also need to get the topicid/logid from the document store. (i hope that's fixxed in the new DB)
• get body of the operation
• load the body into a loro document
• export binary blob from loro
• send it as a new operation using the same topic id as the old one.

(this loses the information who created which operations, but the loro document should still keep author information)