lib/db: Add index to track locally needed files (#4958)

To optimize WithNeed, which is called for the local device whenever an index
update is received. No tracking for remote devices to conserve db space, as
WithNeed is only queried for completion.

lib/model/requests_test.go

@@ -346,7 +346,7 @@ func pullInvalidIgnored(t *testing.T, ft config.FolderType) {
 		expected := map[string]struct{}{invIgn: {}, ign: {}, ignExisting: {}}
 		for _, f := range fs {
 			if _, ok := expected[f.Name]; !ok {
-				t.Fatalf("Unexpected file %v was added to index", f.Name)
+				t.Errorf("Unexpected file %v was added to index", f.Name)
 			}
 			if !f.Invalid {
 				t.Errorf("File %v wasn't marked as invalid", f.Name)