[txindex] transaction Hash -> TxId

Summary:
this updates for current ABC convention, using GetId() instead of
GetHash(), and using the TxId type.

Migration-related types have been left as uint256 since that was how the old
txindex code worked.

Test Plan: make check

Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Reviewed By: jasonbcox, Fabien, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3329