json: fix up msat amounts in non-_msat fields.

We had json_add_amount_msat_only(), which was designed to be used to
print out msat fields, if we had sats.

However, we misused it, so split it into the three different cases:
1. json_add_amount_sat_msat: We are using it correctly, with a field called
   xxx_msat.
2. json_add_amount_sats_deprecated: We were using it wrong, so deprecate
   the old field and create a new one which does end in _msat.
3. json_add_sats: we were using it to hand sats as a JSON parameter to an
   interface, where "XXXsat".

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Deprecated: Plugins: `rbf_channel` and `openchannel2` hooks `their_funding` (use `their_funding_msat`)
Changelog-Deprecated: Plugins: `openchannel2` hook `dust_limit_satoshis` (use `dust_limit_msat`)
Changelog-Deprecated: Plugins: `openchannel` hook `funding_satoshis` (use `funding_msat`)
Changelog-Deprecated: Plugins: `openchannel` hook `dust_limit_satoshis` (use `dust_limit_msat`)
Changelog-Deprecated: Plugins: `openchannel` hook `channel_reserve_satoshis` (use `channel_reserve_msat`)
Changelog-Deprecated: Plugins: `channel_opened` notification `amount` (use `funding_msat`)
Changelog-Deprecated: JSON-RPC: `listtransactions` `msat` (use `amount_msat`)
Changelog-Deprecated: Plugins: `htlc_accepted` `forward_amount` (use `forward_msat`)

tests/utils.py

@@ -189,7 +189,7 @@ def extract_utxos(moves):
             for ev in evs:
                 if ev[0]['vout'] == m['vout']:
                     ev[1] = m
-                    assert ev[0]['output_value'] == m['output_value']
+                    assert ev[0]['output_msat'] == m['output_msat']
                     break
     return utxos