This builds on #10589 (first 5 commits from that PR, last 5 commits are new)
The first couple commits refactor to use the CCoinControl class to pass fee calculation parameters around.
This allows for fixing the buggy interaction in QT between the global payTxFee which can be modified by the RPC call settxfee or temporarily modified by the QT custom fee settings. Before these changes the GUI could sometimes send a transaction with a recently set payTxFee and not respect the settings displayed in the GUI. After these changes, using the GUI does not involve the global transaction confirm target or payTxFee.
The prospective fee displays in the smart fee slider and the coin control dialog are changed to use the fee calculation from GetMinimumFee, this simplifies the code and makes them slightly more correct in edge cases.
Maxing the fee calculation with the mempool min fee is move from estimateSmartFee to GetMinimumFee.
This fixes a long standing bug, and should be tagged for 0.15 as it is holding up finalizing the estimatesmartfee RPC API before release.