pool: Minor refactoring of HsmStorageHandler2

- Don't expose internal queue to other components.

- Make internal stuff private.

- Move utility methods to the correct class.

- Remove unnecessary synchronization

Target: trunk
Require-book: no
Require-notes: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5491/