[performance] Re-arranging maps in config metadata improves startup time

[Aleksey Shipilev]

Hi,

This is rather simple and straight-forward optimization:
http://people.apache.org/~shade/jigsaw/webrev-rearrange-1/

Please take a look. I'd be happy to see this patch landing in Jigsaw
code (are there any other steps besides preparing webrev?).

The change rearranges the hugest map from Map<String(ClassName),
ModuleId> to Map<ModuleId, String(ClassName)> when writing to config
datafile. This helps because module ID is de-duplicated, so it both
occupies less space, and module IDs are not resolved multiple times
during load.

The effect of this is clearly visible on "Hello World" applications:

 BASELINE: 570 +- 20 ms
  PATCHED: 240 +- 20 ms

...which converts to 2.3x improvement. After this patch, loading config
file takes ~30ms. I'm looking if this could be trimmed down further.

-Aleksey.