ticket:8464: Integrate more of Markus' comments.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34865 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Make CacheEntry2 be LRUCache::CacheEntry.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34869 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Added back virtual destructor for RelativeDateTimeData.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34870 251d0590-4201-4cf1-90de-194747b24ca1
ticket:8464: Merge r34822 to r34870 from trunk.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34871 251d0590-4201-4cf1-90de-194747b24ca1