Making stats computed using DiskStatsReporter optional (#17)

* Introduced a flag, which allows IO stats computation when set true

* introducing a new method is DiskStatsReporter which returns the bytes
written at index time

Author: Thejas-bhat

segment.go

@@ -102,8 +102,16 @@ type PostingsIterator interface {
 }
 
 type DiskStatsReporter interface {
+	// BytesRead returns the bytes read from the disk as
+	// part of the current running query.
 	BytesRead() uint64
-	SetBytesRead(uint64)
+	// ResetBytesRead is used by the parent layer
+	// to reset the bytes read value to a consistent
+	// value during operations such as merging of segments.
+	ResetBytesRead(uint64)
+	// BytesRead returns the bytes written to disk while
+	// building an index
+	BytesWritten() uint64
 }
 
 type OptimizablePostingsIterator interface {