Implement hardfork for carrying-capacity.

This patch implements a proposed hardfork that adds a "carrying capacity limit" to the game logic.  The hardfork point as well as the actual capacity limit are still subject to discussion - but the patch can be tested already.  For this, change `FORK_HEIGHT_CARRYINGCAP` in `huntercoin.h` as well as (possibly) the value(s) in `GetCarryingCapacity` in `gamestate.cpp`.  (Near the `FIXME` comments.)

It should at least be confirmed that the client still syncs fine with the patch applied, since the logic to pick up coins has been rewritten and bugs might lead to incomplatible game states even before the fork point is reached.