Resolve "Benchmark the Snapshot code"

These are the results when the current code is run:

BenchmarkSnapshot10MB-4    	    2000	    557676 ns/op
BenchmarkSnapshot100MB-4   	     200	   6556398 ns/op
BenchmarkSnapshot1GB-4     	      20	  64727543 ns/op
BenchmarkSnapshot10GB-4    	       2	 607662104 ns/op

It looks a lot slower than expected but I'm not sure if those results are 100% accurate. In testing builds the modules.SectorSize is a 1/1000th of the one in a release build. That means that the files contain 1000x the sector metadata than they usually would.

The following are the results for creating a snapshot of a file which only contains an empty staticChunks slice without any pieces added to it.

BenchmarkSnapshot10MB-4    	   10000	    180062 ns/op
BenchmarkSnapshot100MB-4   	    1000	   1980898 ns/op
BenchmarkSnapshot1GB-4     	     100	  20717949 ns/op
BenchmarkSnapshot10GB-4    	       5	 220090054 ns/op

Closes #3353 (closed)