Block ingestor doesn't notice if blocks disappear

[defuse]

Steps to reproduce:

1. Have darksidewalletd serve blocks 1000-1003.
2. Modify darksidewalletd's state so that it only has blocks 1000 and 1001.

What actually happens:

• The ingestion algorithm keeps block 1002 and 1003 in the cache and continues serving them even though they're no longer available from "zcashd".

What should happen:

• The ingestion algorithm should notice blocks disappeared and stop serving 1002 and 1003.

I'm not sure if this counts as a "real" bug since I don't think zcashd's block height can ever get shorter like this.