chimera: squash usedFiles() and usedSize() into a single query

kill two birds with one stone

Acked-by: Dmitry Litvintsev
Target: master
Require-book: no
Require-notes: no
(cherry picked from commit 2f921909fa24f16c667a8b7d6855782dd05839aa)
chimera: fix NPE on '.(parent)()' for root inode

Acked-by: Albert Rossi
Acked-by: Dmitry Litvintsev
Target: master, 2.10, 2.9, 2.8, 2.7, 2.6
Require-book: no
Require-notes: no
(cherry picked from commit 82879878110e5254ae1314c759a5fc4f26aecf0c)

Conflicts:
	modules/chimera/src/test/java/org/dcache/chimera/BasicTest.java