HStore synchronized member variable filesCompacting should be private

follow up to HBASE-19728 (which is in an RC for 1.4.2 currently, so no addendums):

the filesCompacting member is currently package private, which allows folks outside of HStore to modify it without ensuring we have hte needed locking in place. it looks like we don't actually use it anywhere outside of HStore, so we should just make it private.