[0.79] Add config option to cause rainfall to very slowly turn flowing water into source blocks

In response to the 0.79 finite water addition, I present a simple config option that causes the strongest flowing water blocks (meta=1) to regenerate into source blocks (meta=0) with a very low chance.

Its a 5% chance to fill/regenerate if the water block is ticked, it could be increased.  As it is now, it regenerates very very slowly.