WIP: p2panda Migration script

[adzialocha]

The gist is:

1. Have a "Reflection protocol version", it's hard-coded somewhere
2. Have a file where we keep that version as well. If it doesn't match the hard-coded version on startup we run some migration code
3. After it has run we're updating the file with the latest version
4. Each migration code is specific to whatever version transition we need