Make sure we have one txdata object per tx we validate. That's required for parallel signature checking.

Summary: In case of parallel validation, we need to make sure we have one txdata object per transaction being validated, or they may end up stepping on each other's toes.

Test Plan:

make check

Enable parallel validation. invalidate block 000000000002f96388721eefe2f3246577636902a7d2c9034ffc9731b1a154cf and then reconsider it. it is accepted. Run the manipulation several times to make sure.

Reviewers: freetrader, sickpig, awemany, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D286