fs: Add directory to optional Purge interface - fixes #1891

- add a directory to the optional Purge interface
- fix up all the backends
- add an additional integration test to test for the feature
- use the new feature in operations.Purge

Many of the backends had been prepared in advance for this so the
change was trivial for them.

backend/b2/b2.go

@@ -1143,7 +1143,8 @@ func (f *Fs) deleteByID(ctx context.Context, ID, Name string) error {
 // if oldOnly is true then it deletes only non current files.
 //
 // Implemented here so we can make sure we delete old versions.
-func (f *Fs) purge(ctx context.Context, bucket, directory string, oldOnly bool) error {
+func (f *Fs) purge(ctx context.Context, dir string, oldOnly bool) error {
+	bucket, directory := f.split(dir)
 	if bucket == "" {
 		return errors.New("can't purge from root")
 	}
@@ -1218,19 +1219,19 @@ func (f *Fs) purge(ctx context.Context, bucket, directory string, oldOnly bool)
 	wg.Wait()
 
 	if !oldOnly {
-		checkErr(f.Rmdir(ctx, ""))
+		checkErr(f.Rmdir(ctx, dir))
 	}
 	return errReturn
 }
 
 // Purge deletes all the files and directories including the old versions.
-func (f *Fs) Purge(ctx context.Context) error {
-	return f.purge(ctx, f.rootBucket, f.rootDirectory, false)
+func (f *Fs) Purge(ctx context.Context, dir string) error {
+	return f.purge(ctx, dir, false)
 }
 
 // CleanUp deletes all the hidden files.
 func (f *Fs) CleanUp(ctx context.Context) error {
-	return f.purge(ctx, f.rootBucket, f.rootDirectory, true)
+	return f.purge(ctx, "", true)
 }
 
 // copy does a server side copy from dstObj <- srcObj