Refactor fee rules to make them actually readable.

This (nearly) doesn't change fee rules at all:
 * To make it into the fee transaction area, the dPriority comparison
   changed from < to <=
 * We now just ignore transactions > MAX_BLOCK_SIZE/4 instead of
   doing some calculations to require increasingly large fees as
   size increases.
Merge pull request #2660 from TheBlueMatt/gmfrefactor

Refactor fee rules to make them actually readable.