Bug 4823: assertion failed: "lowestOffset () <= target_offset" (#370)

Bug 4823: assertion failed: "lowestOffset () <= target_offset" (#370)

This assertion could be triggered by various swapout failures for
ufs/aufs/diskd cache_dir entries.

The bug was caused by 4310f8b change related to storeSwapOutFileClosed()
method. Before that change, swapout failures resulted in
StoreEntry::swap_status set to SWAPOUT_NONE, preventing
another/asserting iteration of StoreEntry::swapOut().

This fix adds SWAPOUT_FAILED swap status for marking swapout failures
(instead of reviving and abusing SWAPOUT_NONE), making the code more
reliable.

Also removed storeSwapOutFileNotify() implementation.  We should not
waste time on maintaining an unused method that now contains conflicting
assertions: swappingOut() and !hasDisk().