pool: Let DoorTransferFinished and PoolFileFlushed use FileAttributes

We are slowly moving away from StorageInfo. This patch moves two
pool notification messages to FileAttributes, although it is still
used to transport StorageInfo.

Care is taken to maintain backwards compatibility with old pools
(two way for DoorTransferFinished as it may be sent between pools).

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5323/