Diskpacked: fill holes.

Add holes freelist and fill as possible.
Hole merging is missing, I'd need a working sorted.KeyValue.Find,
but seems I don't understand the usage of it, or I'm using bad keys
- does not work as intended: see FIXME in dele.go

Change-Id: I2d67a2cab30a1da10f640c26d046de4d5ef0f8a2