pool: Clean up flush controller

- Flush properties can now be queried through 'bean properties'

- Shell commands are now implemented as annotated commands

- Background tasks now execute on a thread pool

- Addresses inconsistent source code formatting

- Addresses potential lock contention on flush controller monitor

Target: trunk
Require-book: no
Require-notes: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5467/