previously load_transactions() had to be called before upgrade();
now we reverse this order.
to reproduce/illustrate issue, before this commit:
try running convert_version_17 and convert_version_18
(e.g. see testcase test_upgrade_from_client_2_9_3_old_seeded_with_realistic_history)
and then in qt console:
>> wallet.storage.db.get_data_ref('spent_outpoints') == wallet.storage.db.spent_outpoints
False
>> wallet.storage.db.get_data_ref('verified_tx3') == wallet.storage.db.verified_tx
False
