refactor: replace pointers by references within tx_verify.{h,cpp}

Summary:

affects "prevHeights" parameter of the functions
- CalculateSequenceLocks()
- SequenceLocks()

Backport of core PR19069.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8478