Add a field to show if an account already existed in v1

Account table has no creation date information. We could add a simple boolean field existed_in_v1 to track if it existed in v1 or not, or better a numeric field created_on to track block number of account creation (better to sort accounts for example).

See discussion https://forum.duniter.org/t/g1nkgo-v2-support-progress-plan-and-pubkeys-address-proposal/12617

In the same way, we could have a last_touched field like the lastChange field of identity and smith.