Don't allow getutxo answer to go over the size of the send buffer.

Limit the possible answer size for getutxo.

This does not adjust MAX_INV_SZ because it's unnecessary for the mix, additionally, requesting normal outputs even up to MAX_INV_SZ does not run out of space in the send buffer. To make a giant response you need a giant output. So there might be some use case for doing a big query, at any rate, if the send buffer is too big then that's a general issue we should fix elsewhere.

In the end I didn't truncate the answer. I just return false which hits Misbehaving(). The reason is, again, no real app will ever hit this case. You need to deliberately pick a giant output and a giant number of repeated queries. So Misbehaving is a reasonable solution.
You can use:
```
bytes_ued += ::GetSerializeSize(coin, SER_NETWORK, PROTOCOL_VERSIO);
```
instead of defining a custom size computer.