mempool: Save & restore mempool entry acceptance height on reorg
Summary:
MR !1128 introduced a regression whereby mempool entry acceptance height would be lost across reorgs and just set to the new tip height after the block unwind is done. This MR fixes that and restores previous behavior where a mempool entry's acceptance height is remembered until a node restart (at which point it is lost since it is not serialized). The `mempool_reorg.py` functional test was modified to also test that the height is preserved properly across reorgs.
Port of bchn#1162. Depends on D13133.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D13141