backpack account primary public key derivation path is unbounded

[tomlinton]

A user can theoretically set their primary public key to any derivation path, but we can only provide a fixed set of recovery paths to search for the primary public key.

If a BIP44 path is:

m / purpose' / coin_type' / account' / change / address_index

• One potential way of dealing with this is storing the account index of the primary key on the server so we know where to search, but the address_index is still unbounded.
• Store the complete derivation path which doesn't seem great.
• We could store the account and address_index, but that is effectively the same as storing the complete path.

Related #1716