In \crossref{blockchain}, define what a settled network upgrade is, specify requirements for checkpointing, and allow nodes to impose a limitation on rollback depth. Also in \crossref{bctv}, note that this checkpointing requirement mitigates the risks of not performing BCTV14 zk proof verification.
In \crossref{blockchain}, define what a settled network upgrade is, specify requirements
for checkpointing, and allow nodes to impose a limitation on rollback depth. Also in
\crossref{bctv}, note that this checkpointing requirement mitigates the risks of not
performing BCTV14 zk proof verification.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips