ZIP 308: Migration transactions MUST have an expiry delta of 450 blocks.
ZIP 308: Migration transactions MUST have an expiry delta of 450 blocks.

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

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