Correctly integrate CNoteData::witnessHeight into wallet code

[str4d]

Closes #1715.

[bitcartel]

• Maybe constant instead of magic number -1.
• Perhaps move assert(nWitnessCacheSize > 0); to be under nWitnessCacheSize -= 1; so that it appears before loop to // Check the validity of the cache
• Implement this? // TODO: If nWitnessCache is zero, we need to regenerate the caches (Rebuild witness and nullifier caches in wallet if a large reorganisation occurs #1302)

[str4d]

• The -1 is taken from the implicit "no chain" height in CChain::Height(), which by definition doesn't have a defined constant. Perhaps it would be better to just use < 0 instead, although had I done so before, the assertion wouldn't have triggered and we wouldn't have identified this bug (although technically, maybe it wouldn't have been a problem then - but the other bugs we found along the way definitely are).
• It's somewhat cosmetic (as is the loop that got added in between for sanity checking), so I'd be inclined to leave it as-is for the purpose of making the diff easier to read. We can move it when implementing Rebuild witness and nullifier caches in wallet if a large reorganisation occurs #1302.
• No, that should be in a separate PR (at the very least, it would not be implementable for 1.0.3).

[ebfull]

ACK

@zkbot r+

[zkbot]

📌 Commit a4ef3aa has been approved by ebfull

[zkbot]

⌛ Testing commit a4ef3aa with merge 4e4ca6d...

[zkbot]

☀️ Test successful - zcash

[bitcartel]

@ebfull I didn't ACK this yet, still reviewing and looking at tickets #1715 and #1794. @str4d I'll continue to add any comments to this ticket and if we need to make any changes for release we can open a new PR.