txout nonce is inconsistently stored in coinsview

[apoelstra]

We currently do not de/serialize the txout nonce in the Coin class, meaning that after restarting the node, all nonces in the utxoset are cleared, resulting in a network where everyone has an (inconsequentially) different version of the UTXO set.

Currently the nonce is not used for anything consensus-critical, but it may be in the future --- for example, we might want input nonces covered by a future sighash version --- so this is a massive footgun.

[roconnor-blockstream]

I recommend going through and explicitly censoring the nonce stored in any CoinsView that represents the UTXO set.

[apoelstra]

I think we're going to just fix the serialization (after confirming that the rescan is not horribly long)

[stevenroose]

Addressed by #1008.