set nNonce to Null in CoinsView

[apoelstra]

Alternative to #1006

Fixes #1008

[sanket1729]

utACK a70ff3c

[apoelstra]

Lol, some of the new assertions are triggering in CI

[apoelstra]

Sigh, so, the CCoinsView is a base class of two things -- the "UTXO set" which is saved to disk in the coinstate/ directory, where we want to erase the nonce, and the mempool tx db, where we do not want to erase the nonce (and there are existing assertions that prevent us from doing so).

[stevenroose]

Any update on solving this?

[roconnor-blockstream]

Why don't we want to erase it from the mempool?

[apoelstra]

@stevenroose no update. I think it might be practically impossible.

@roconnor-blockstream because of tangled inheritance relationships between various mempool-like structures. If you do what you're suggesting you will get far away assertion failures in consensus code where they might be meaningful.

[apoelstra]

I think we should just live with this bug.