ticket:10461: Proof of concept phase 2. Add NumberingSystemCache. DecimalFormat creation down from 44us to 7.7us. DecimalFormat cloning down from 3.4us to 1.05us. This phase does not affect cloning as NumberingSystem objects are used as temporary objects when creating DecimalFormat objects.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34570 251d0590-4201-4cf1-90de-194747b24ca1