reduce number of calls tagOnChain()

Before this patch, tagOnChain is called rack_count * object_etags_count * 2(in
most cases). In my testing was 3000 racks and 5 etags on object. I ran
10 rounds of renderRackSpaceForObject() and get 7.03 sec before, and
6.32 after patching.
Merge pull request #78 from gescheit/perf2

reduce number of calls tagOnChain()