Access session keys by account only

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we have some duality in the DB that stores session keys, like in #3671 (comment).

Describe the solution you'd like

1. Eliminate user-defined keys from the DB interface, put the key and derive an account from it internally, get by account only.
2. For session tokens v1 account can be derived from the session key embedded in the token itself, UUID just isn't needed.
3. Old keys need to be migrated on node upgrade, but that's rather easy.

Describe alternatives you've considered

Bloating the DB doesn't make much sense, so we better just stop it.

Additional context

#3671