Update mempool's mapDelta to use TxId

Summary:
Using TxId instead of uint256 makes the API more precise.

Depends on D4598

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4599