Don't store encrypted seed entropy

[mnzaki]

Description

Don't store the seed entropy, even if encrypted (password is in OS keystore). Instead store the derived key pairs directly. This will provide for added security (phone compromise is not necessarily a seed compromise any more). It will also make #1427 trivial

Current idea is to use the persona table to store the generated keys (encrypting the privateKey of course) and DID.

[chunningham]

@mnzaki this is something we should look at in the typeorm sdk storage stuff

[mnzaki]

Is this even in the notion backlog?