Check v2 reserves after funding_contribution_satoshis is applied

We check this when validating `splice_init`, `splice_ack` messages, and
also when validating user-specified contributions.

From BOLT 2:
```
- If `funding_contribution_satoshis` is negative and its absolute value
  is greater than the sending node's current channel balance:
  - MUST send a `warning` and close the connection or send an `error`
    and fail the channel.
```

and further down:
```
If a side does not meet the reserve requirements, that's OK: but if they
take funds out of the channel, they must ensure that they do meet them.
If your peer adds a massive amount to the channel, then you only have
to add more reserve if you want to contribute to the splice (and you
can use `tx_remove_output` and/or `tx_remove_input` part-way through if
this happens).
```

Therefore, we check the v2 reserve anytime
`funding_contribution_satoshis` is not equal to zero.

We allow parties to draw from their previous reserve, as long as they
satisfy their v2 reserve.

Author: tankyleo