Fix problem with huge input data generated with phygthmap.
Implement two different versions of SparseLong2ShortMapFunction. One is very memory effective, 
the other has almost no limits regarding the number of ids that can be stored but needs more
heap. If the max heap value is > 2GB the program uses the latter structure.

git-svn-id: http://svn.mkgmap.org.uk/splitter/trunk@301 838fabe5-b1a8-4b81-9896-7a84386980e9