Make float <-> int casts explicit outside of test, qt, CFeeRate

Summary:
Backport of core PR11303
Only the commit 53a6590 is applicable for us.
https://github.com/bitcoin/bitcoin/pull/11303/commits/53a6590f496b25174c740927243bf8307541b0b9

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3313