Use TxId in CWallet::mapTx

Summary:
As per title

Depends on D1689, D1688 and D1694

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1695