`listfunds` does not take in-flight HTLCs into account

### Issue and Steps to Reproduce

1. Initiate a payment via `pay`
2. Asynchronously to the `pay` call `listpeerchannels` and `listfunds` (in this order)
3. Notice a disagreement.

The `listfunds` `our_amount_msat` field may be more than the `listpeerchannels` `spendable_msat` field, as the latter takes the in-flight HTLC into account, while the former does not:

`listpeerchannels`:
```javascript
{
'channels': [
{
'peer_id': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518',
'peer_connected': True,
'channel_type': {'bits': [12], 'names': ['static_remotekey/even']},
'state': 'CHANNELD_NORMAL',
'scratch_txid': 'a021ea7fcb80b95b21662aa3981efba18a15bff2a524cc64324b8af2c4755489',
'last_tx_fee_msat': 7964000msat,
'feerate': {'perkw': 11000, 'perkb': 44000},
'owner': 'channeld',
'short_channel_id': '103x1x0',
'direction': 0,
'channel_id': '72fdc31491364795fd686c624785ce1003dd29092b17f35dfffdd583325e7061',
'funding_txid': '61705e3283d5fdff5df3172b0929dd0310ce8547626c68fd9547369114c3fd72',
'funding_outnum': 0,
'close_to_addr': 'bcrt1p2au6qc8jqr2qaru8rfqwhytpfnzmlfks5hu99czngq98lmlcv9gqy9ctvg',
'close_to': '51205779a060f200d40e8f871a40eb91614cc5bfa6d0a5f852e053400a7feff86150',
'private': False,
'opener': 'remote',
'alias': {'local': '2528304x15270177x39405', 'remote': '6952102x16556827x19020'},
'features': ['option_static_remotekey'],
'funding': {'local_funds_msat': 0msat, 'remote_funds_msat': 1000000000msat, 'pushed_msat': 0msat},
'to_us_msat': 900000000msat,
'min_to_us_msat': 0msat,
'max_to_us_msat': 900000000msat,
'total_msat': 1000000000msat,
'fee_base_msat': 1msat,
'fee_proportional_millionths': 10,
'dust_limit_msat': 546000msat,
'max_total_htlc_in_msat': 18446744073709551615msat,
'their_reserve_msat': 10000000msat,
'our_reserve_msat': 10000000msat,
'spendable_msat': 888000000msat,
'receivable_msat': 66504000msat,
'minimum_htlc_in_msat': 0msat,
'minimum_htlc_out_msat': 0msat,
'maximum_htlc_out_msat': 990000000msat,
'their_to_self_delay': 5,
'our_to_self_delay': 5,
'max_accepted_htlcs': 483,
'state_changes': [{'timestamp': '2023-07-17T11:39:26.615Z', 'old_state': 'CHANNELD_AWAITING_LOCKIN', 'new_state': 'CHANNELD_NORMAL', 'cause': 'remote', 'message': 'Lockin complete'}],
'status': ['CHANNELD_NORMAL:Channel ready for use.'],
'in_payments_offered': 1,
'in_offered_msat': 900000000msat,
'in_payments_fulfilled': 1,
'in_fulfilled_msat': 900000000msat,
'out_payments_offered': 1,
'out_offered_msat': 1000000msat,
'out_payments_fulfilled': 0,
'out_fulfilled_msat': 0msat,
'htlcs': [
{'direction': 'out', 'id': 1, 'amount_msat': 1000000msat, 'expiry': 109, 'payment_hash': '9e1a49330b1bd6d5eb2ea841ebc121878e0d5c2c512d8fe8f99767402a4e0a94', 'state': 'SENT_ADD_HTLC', 'local_trimmed': True},
{'direction': 'out', 'id': 0, 'amount_msat': 1000000msat, 'expiry': 109, 'payment_hash': '9bf332e4569c67945bb48d44786ffba260e3d2010d35b39c700248c15877bd93', 'state': 'SENT_ADD_COMMIT', 'local_trimmed': True}
]
}
]
}
```

`listfunds` called after the above `listpeerchannels`:

```javascript
{
'outputs': [],
'channels': [
{
'peer_id': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518',
'connected': True,
'state': 'CHANNELD_NORMAL',
'channel_id': '72fdc31491364795fd686c624785ce1003dd29092b17f35dfffdd583325e7061',
'short_channel_id': '103x1x0',
'our_amount_msat': 900000000msat,
'amount_msat': 1000000000msat,
'funding_txid': '61705e3283d5fdff5df3172b0929dd0310ce8547626c68fd9547369114c3fd72',
'funding_output': 0
}
]
}
```

### Reproduction test

```python
def test_listfunds_vs_listpeers(node_factory, executor):
"""Reproduce an inconsistency in `listpeers` vs `listfunds`

`listfunds` appears not to consider HTLCs that are still pending
while `listpeers` does.
"""
l2, l1 = node_factory.line_graph(2)

# Rebalance, because the fundee is the one doing the calls here
i = l1.rpc.invoice(
10**8 * 9,
'rebalance',
"Rebalance since we check fundee"
)['bolt11']
l2.rpc.pay(i)

import time
time.sleep(1)

reserve = 10**7

lf = l1.rpc.listfunds()['channels'][0]['our_amount_msat']
lp = l1.rpc.listpeerchannels()['channels'][0]['spendable_msat']

# We start out with both agreeing
assert lf == lp + reserve

from rich.pretty import pprint
for i in range(0, 100):
i = l2.rpc.invoice(10**6, f"test{i}", "test")['bolt11']
f = executor.submit(l1.rpc.pay, i)

# Monotonicity: lf and lp monotonically decrese because l1 is
# sending. The later call of the two needs to be equal or
# lower than the earlier call, no flip-flopping allowed
c = l1.rpc.listpeerchannels()
pprint(c)
lpn = c['channels'][0]['spendable_msat']

# According to the report, calling `listfunds` shortly after
# `listpeers` will not have the updated `our_amount_msat`.
f = l1.rpc.listfunds()
pprint(f)
lfn = f['channels'][0]['our_amount_msat']

assert lpn <= lp
assert lfn <= lf
assert lfn <= lp + reserve
lp = lpn
lf = lfn
time.sleep(0.1)
```

### Discussion

The accounting on in-flight HTLCs isn't as clear cut as many would believe. Essentialy an in-flight HTLC may either be counted towards the recipient (assuming it'll succeed) or the sender (assuming it'll fail), and we may be in this state for long periods (slow or stuck HTLCs). While we generally do not guarantee consistency among multiple RPC calls, this case is due to these different interpretations, so we should likely pick one and stick with it. Since `listpeerchannels` is used for routing decisions it has to take the pessimistic view as the funds in HTLCs are not available for new HTLCs.

- [ ] Do we want to align the two methods, and subtract HTLCs from `listfunds` `our_amount_msat`?