lib/versioner: Reduce surface area (#6186)

* lib/versioner: Reduce surface area

This is a refactor while I was anyway rooting around in the versioner.
Instead of exporting every possible implementation and the factory and
letting the caller do whatever, this now encapsulates all that and
exposes a New() that takes a config.VersioningConfiguration.

Given that and that we don't know (from the outside) how a versioner
works or what state it keeps, we now just construct it once per folder
and keep it around. Previously it was recreated for each restore
request.

* unparam

* wip

lib/versioner/simple_test.go

@@ -41,7 +41,7 @@ func TestTaggedFilename(t *testing.T) {
 		}
 
 		// Test parser
-		tag := ExtractTag(tc[2])
+		tag := extractTag(tc[2])
 		if tag != tc[1] {
 			t.Errorf("%s != %s", tag, tc[1])
 		}
@@ -61,7 +61,7 @@ func TestSimpleVersioningVersionCount(t *testing.T) {
 
 	fs := fs.NewFilesystem(fs.FilesystemTypeBasic, dir)
 
-	v := NewSimple("", fs, map[string]string{"keep": "2"})
+	v := newSimple(fs, map[string]string{"keep": "2"})
 
 	path := "test"