Separate wallet key for each connection

[frnandu]

This will allow better privacy between connections, which currently can see metadata from events of other connections since it uses the same wallet key.

[frnandu]

Some considerations:

• Currently the single wallet key is kept in DB encrypted and when hub starts and is unlocked with password it is decrypted and that private key is kept in memory, so that when needed for signing events with wallet priv key it is accessible.
• If we would want to have a separate wallet key for each connection, we would need to keep each wallet private key in App model in DB, ideally encrypted so that if db leaks it will not be a problem.
• In order to be able to encrypt a new wallet private key each time a new connection is created we would need the unlock password so that we can encrypt the new wallet private key and write it to the DB encrypted.
• AFAIK unlock password is not (and should not be) kept in memory, so this presents a problem.
• Should we write wallet private keys for each connection in App DB model unencrypted???
• Any other ideas how to implement this in a secure manner?
  @rolznz @bumi @reneaaron @im-adithya