Reduces garbage left behind in /tmp

o Also fixes an issue where a FileChannel was kept open if
  StoreLocker#checkLock failed in tryLock, preventing files from being
  deleted on Windows.
o RRD file location when impermanent db was supplied as a directory, which
  made opening it always fail and create one extra "rrd-invalid"
  file/directory that the tests didn't clean up.
o Generally making sure tests that create temp file deletes them in @After