current BlackHole de-dup scheme

- get current hash H1 for this sector number

- new sector OR replacing data?
	- replacing data?
		- get number of references to this hash C1
		- decrease that count C1
		- new count C1 == 0?
			- delete hash H1/data pair
			- delete usage counter C1
		else
			- set new usage count C1

		- get number of references to the new hash C2
		- increase that count C2
		- new count C2 == 1?
			- store hash H2 /data pair
		- set new usage count C2

- map sector number to new hash H2