error in the wild: Found a later subchain with eldest kid

[maxtaco]

See keybase/keybase-issues#1769

[oconnor663]

The server needs to enforce that we don't reuse eldest keys. (Bit of a hack, but it'll matter less when most eldest keys are device keys.)

[oconnor663]

Server-side fix: https://github.com/keybase/keybase/pull/478

[oconnor663]

As part of this, we need to update the node client to go down the is_update codepath on key/add.json.

[oconnor663]

I was wrong, no client change is needed. This is done. (Just confirmed by creating an account with the node client, making a few sigs, running keybase revoke to nuke everything, and then trying to keybase push the same eldest key again.)