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.
add a network chassis to the dictionary
Merge pull request #78 from gescheit/perf2

reduce number of calls tagOnChain()
also add some modules for the chassis
refine commit 736acd8

* getPrevIDforRack(): this function had been in the source code for a
  long time, for less painful migration add it back as a wrapper around
  the new, fast implementation until 0.21.0
* getNextIDforRack(): idem
* renderRack(): array_fetch() has no purpose in this context
Merge https://github.com/RackTables/racktables into maintenance-0.20.x

Conflicts:
	wwwroot/inc/dictionary.php