Separate reason for premature spends (coinbase/locktime)

Summary:
This is a partial backport of Core PR15141 : https://github.com/bitcoin/bitcoin/pull/15141/commits/0ff1c2a838da9e8dc7f77609adc89124bbea3e2b

Depends on D6742

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6743