walletdb: chan dict: small clean-up (incl db upgrade)

- "fail_htlc_reasons" was removed in 9b1c40e396
- "unfulfilled_htlcs": rm 2 dead items from the 4-tuple,
   and convert False value of forwarding_key

tests/test_lnchannel.py

@@ -108,7 +108,6 @@ def create_channel_state(funding_txid, funding_index, funding_sat, is_initiator,
             'data_loss_protect_remote_pcp': {},
             'state': 'PREOPENING',
             'log': {},
-            'fail_htlc_reasons': {},
             'unfulfilled_htlcs': {},
             'revocation_store': {},
             'channel_type': lnutil.ChannelType.OPTION_STATIC_REMOTEKEY