freezer: disable compression on hashes and difficulties (#14)

* freezer: disable compression on hashes and difficulties

* core/rawdb: address review concerns

* core/rawdb: address review concerns

core/rawdb/schema.go

@@ -80,6 +80,16 @@ const (
 	freezerDifficultyTable = "diffs"
 )
 
+// freezerNoSnappy configures whether compression is disabled for the ancient-tables.
+// Hashes and difficulties don't compress well.
+var freezerNoSnappy = map[string]bool{
+	freezerHeaderTable:     false,
+	freezerHashTable:       true,
+	freezerBodiesTable:     false,
+	freezerReceiptTable:    false,
+	freezerDifficultyTable: true,
+}
+
 // LegacyTxLookupEntry is the legacy TxLookupEntry definition with some unnecessary
 // fields.
 type LegacyTxLookupEntry struct {