Improvements to TileEntityRift

Made some improvements to TileEntityRift. The main reason for these
changes was to remove the field nearestRift - we should not hold on to
references to links. Now we simply track the location of the nearest
rift. I also confirmed that closeRift() and updateNearestRift() must be
allowed to run on both the client and the server. If the client doesn't
run those functions, then adjacent rifts don't connect as expected and
the rift removal animation doesn't work.