Get space that can be reclaimed (#189)

* get reclaimable space added

* import order fix

Co-authored-by: yash <yash.chandra@grabpay.com>

internal/data/codec/encoder.go

@@ -13,6 +13,7 @@ const (
 	keySize      = 4
 	valueSize    = 8
 	checksumSize = 4
+	MetaInfoSize = keySize + valueSize + checksumSize
 )
 
 // NewEncoder creates a streaming Entry encoder.