fix(dedupe): use a common blobs dir to simplify

Currently, our dedupe scheme is very complicated since master copy is
kept in one of the repo dirs and if that image is deleted, the owning
repo is changed, requiring multiple repo locks causing unnecessary
contention.

Author: rchincha

pkg/storage/imagestore/imagestore.go

@@ -633,7 +633,7 @@ func (is *ImageStore) PutImageManifest(repo, reference, mediaType string, //noli
 	}
 
 	// write manifest to "blobs"
-	dir := path.Join(is.rootDir, repo, ispec.ImageBlobsDir, mDigest.Algorithm().String())
+	dir := path.Join(is.rootDir, "_blobs", mDigest.Algorithm().String())
 	manifestPath := path.Join(dir, mDigest.Encoded())
 
 	binfo, err := is.storeDriver.Stat(manifestPath)
@@ -1017,7 +1017,7 @@ func (is *ImageStore) FinishBlobUpload(repo, uuid string, body io.Reader, dstDig
 		return zerr.ErrBadBlobDigest
 	}
 
-	dir := path.Join(is.rootDir, repo, ispec.ImageBlobsDir, dstDigest.Algorithm().String())
+	dir := path.Join(is.rootDir, "_blobs", dstDigest.Algorithm().String())
 
 	err = is.storeDriver.EnsureDir(dir)
 	if err != nil {
@@ -1260,7 +1260,8 @@ func (is *ImageStore) DeleteBlobUpload(repo, uuid string) error {
 
 // BlobPath returns the repository path of a blob.
 func (is *ImageStore) BlobPath(repo string, digest godigest.Digest) string {
-	return path.Join(is.rootDir, repo, ispec.ImageBlobsDir, digest.Algorithm().String(), digest.Encoded())
+	repo = "_blobs"
+	return path.Join(is.rootDir, repo, digest.Algorithm().String(), digest.Encoded())
 }
 
 func (is *ImageStore) GetAllDedupeReposCandidates(digest godigest.Digest) ([]string, error) {