ticket:10461: Introduce sharedobjectptr.h and make MeasureFormat use it.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@35079 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10461: MeasureFormat no longer keeps its own cache of NumberFormat and PluralRules.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@35081 251d0590-4201-4cf1-90de-194747b24ca1