Fix fee estimate rounding errors

Summary:
Fixes an issue that has been brought up where fees were truncated down during estimation,
causing the estimatefee rpc to return values less than the current minimum relay feerate.

We replace default truncation with mathimatical ceilings everywhere relevant for estimating
a fee to be paid. Note well: we leave all minimum calculation alone so as to not underbill
by accident.

Builds on D1670

Inspired by Core PR 11303 by Matt Corallo.

Test Plan:

make VERBOSE=1 check && ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1815