channeldb + wtdb: specify freelist bbolt options by default

This commit specifies two bbolt options when opening the underlying
channel and watchtower databases so that there is reduced heap
pressure in case the bbolt database has a lot of free pages in the
B+ tree.

watchtower/wtdb/db_common.go

@@ -63,7 +63,14 @@ func createDBIfNotExist(dbPath, name string) (*bbolt.DB, bool, error) {
 		}
 	}
 
-	bdb, err := bbolt.Open(path, dbFilePermission, nil)
+	// Specify bbolt freelist options to reduce heap pressure in case the
+	// freelist grows to be very large.
+	options := &bbolt.Options{
+		NoFreelistSync: true,
+		FreelistType:   bbolt.FreelistMapType,
+	}
+
+	bdb, err := bbolt.Open(path, dbFilePermission, options)
 	if err != nil {
 		return nil, false, err
 	}