[mempool] remove use of deprecated estimateSmartPriority function

Summary: As per title. Depends on D1352, D1353, D1351

Test Plan:

ninja check && ./test/functiona/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1354