ticket:10461: Preliminary proof of concept. Cache DecimalFormatSymbols only within DecimalFormat. Resulted in a 3X increase in creation of DecimalFormat. 44us down to 14.3us when creating from scratch; 3.4us down to 1.02us when cloning

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34568 251d0590-4201-4cf1-90de-194747b24ca1