rework help messages for fee-related options

- mention the units and show the default for -paytxfee
http to https, 2013 to 2014
typedef std::map<uint256, CCoins> to CCoinsMap

This makes it possible to switch to a more efficient map type
without changing all occurences manually.

Merges half of #4413.
Merge pull request #4447

9a6497e http to https, 2013 to 2014 (Trevin Hofmann)
Merge pull request #4325

4aaa017 rework help messages for fee-related options (Philip Kaufmann)