ticket:10461: In SharedPtr adopt to adoptInstead(). Make sure get() in LRUCache increments reference count while holding mutex. Add safeguards in case subclasses of LRUCache implement create() incorrectly. Added -> and (*) operators to sharedPtr. Disallow new with shared pointer. Add == and != for raw pointers so that sharedPtr == NULL still works. Make sure identity assignmen works. Replaced calls to readOnly() operator-> and ==NULL.

git-svn-id: http://source.icu-project.org/repos/icu/icu/branches/tkeep@34573 251d0590-4201-4cf1-90de-194747b24ca1