[amount] Extend GetFee() by optional flag ceil #7660

• This flag causes rounding up to the next satoshi instead of just truncating (default behavior)
• This reverts the hard-to-read and buggy code introduced in d88af560111863c3e9c1ae855dcc287f04dffb02 (#4465)

master happily produces the following results:

    feeRate = CFeeRate(123);
    BOOST_CHECK_EQUAL(feeRate.GetFee(0), 123);   // TODO: Should be 0
    BOOST_CHECK_EQUAL(feeRate.GetFee(8), 123);   // TODO: Should be 1
    BOOST_CHECK_EQUAL(feeRate.GetFee(9), 1);     // TODO: Should be 2
    BOOST_CHECK_EQUAL(feeRate.GetFee(121), 14);  // TODO: Should be 15
    BOOST_CHECK_EQUAL(feeRate.GetFee(122), 15);  // TODO: Should be 16
    BOOST_CHECK_EQUAL(feeRate.GetFee(999), 122); // TODO: Should be 123

@morcos suggested an alternative would be to use std::ceil in all cases. This also changes the dust value by a small amount (fa03771), so I am not sure if this really is preferred?

I don't know what is the expected usage, but wouldn't it be simpler to just add a CFeeRate::GetFeeSatoshis(size) method ? Maybe that would be too disruptive for the indended use, I don't really know.

EDIT: Although it may look unrelated, maybe this would also a good opportunity to move CFeeRate, CTxOut::GetDustThreshold() and CTxOut::IsDust() finally out of libconsensus. Something like https://github.com/jtimon/bitcoin/commit/dc1b96d368d471ea5242ca021148f9f900f69661 or #5114 but maybe not based on #6068, since #6068 seems extremely hard for reasons I still don't understand. I guess I just leave it there in case someone wants to rebase or rewrite such a thing. Sorry, I cannot help remembering every time I review changes in amount.o.

Closing this per IRC discussion.