To make sure we don't create any manual DB transactions, we refactor the
ImportAccountDryRun method to use walletdb.Update and the new
walletdb.ErrDryRunRollBack error for making sure a rollback is issued.
This method returns the first N external and internal addresses, which
can be presented to users to confirm whether the account has been
imported correctly.
Simnet was previously left out as it didn't have defined HD versions for
some of our key scopes. To allow testing importing accounts into simnet
wallets, we fall back to use the mainnet HD versions.
This commit also addresses an issue with simnet wallets that would arise
whenever ScopedKeyManager.AccountProperties was invoked:
`failed to retrieve account public key: unsupported net SimNet`
