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.

internal/metadata/metadata.go

@@ -7,7 +7,8 @@ import (
 )
 
 type MetaData struct {
-	IndexUpToDate bool `json:"index_up_to_date"`
+	IndexUpToDate    bool  `json:"index_up_to_date"`
+	ReclaimableSpace int64 `json:"reclaimable_space"`
 }
 
 func (m *MetaData) Save(path string, mode os.FileMode) error {