diskpacked: implement CreateQueue

as a sub - localdisk.DiskStorage, because it handles temporary usage
(deletions) optimally.

Change-Id: I94ea723c27a4924a36016b916a1e3028efcc0949
Diskpacked: support deletion

Delete from index and zero out blob's data.
Use FALLOC_FL_PUNCH_HOLE if available (linux >= 2.6.38).

Change-Id: I0c194fd6734a35b520212398d93dfdc5cabadeb5
Merge branch 'dp-delete'

Conflicts:
	pkg/blobserver/diskpacked/diskpacked.go

Change-Id: Id84a4169c7d4bf30553728deec545eb6fc194fde
Merge branch 'dp-idx-rebuild'
Merge branch 'gensync'