fix features, add mers feature, improve merscfg, add run-mers mode

musicdb-lib/src/data/database.rs

@@ -1,6 +1,5 @@
 use std::{
-    collections::{BTreeSet, HashMap, HashSet},
-    convert::identity,
+    collections::{BTreeSet, HashMap},
     fs::{self, File},
     io::{BufReader, Read, Write},
     path::PathBuf,