More changes for registry-v2 in python.

Implement the minimal changes to the local filesystem storage driver and feed them through the distributed storage driver.
Create a digest package which contains digest_tools and checksums.
Fix the tests to use the new v1 endpoint locations.
Fix repository.delete_instance to properly filter the generated queries to avoid most subquery deletes, but still generate them when not explicitly filtered.

digest/digest_tools.py

@@ -0,0 +1,62 @@
+import re
+import os.path
+import hashlib
+
+from collections import namedtuple
+
+
+Digest = namedtuple('Digest', ['is_tarsum', 'tarsum_version', 'hash_alg', 'hash_bytes'])
+
+
+DIGEST_PATTERN = r'(tarsum\.(v[\w]+)\+)?([\w]+):([0-9a-f]+)'
+DIGEST_REGEX = re.compile(DIGEST_PATTERN)
+
+
+class InvalidDigestException(RuntimeError):
+  pass
+
+
+def parse_digest(digest):
+  """ Returns the digest parsed out to its components. """
+  match = DIGEST_REGEX.match(digest)
+  if match is None or match.end() != len(digest):
+    raise InvalidDigestException('Not a valid digest: %s', digest)
+
+  is_tarsum = match.group(1) is not None
+  return Digest(is_tarsum, match.group(2), match.group(3), match.group(4))
+
+
+def content_path(digest):
+  """ Returns a relative path to the parsed digest. """
+  parsed = parse_digest(digest)
+  components = []
+
+  if parsed.is_tarsum:
+    components.extend(['tarsum', parsed.tarsum_version])
+
+  prefix = parsed.hash_bytes[0:2].zfill(2)
+  components.extend([parsed.hash_alg, prefix, parsed.hash_bytes])
+
+  return os.path.join(*components)
+
+
+def sha256_digest(content):
+  """ Returns a sha256 hash of the content bytes in digest form. """
+  def single_chunk_generator():
+    yield content
+  return sha256_digest_from_generator(single_chunk_generator())
+
+
+def sha256_digest_from_generator(content_generator):
+  """ Reads all of the data from the iterator and creates a sha256 digest from the content
+  """
+  digest = hashlib.sha256()
+  for chunk in content_generator:
+    digest.update(chunk)
+  return 'sha256:{0}'.format(digest.hexdigest())
+
+
+def digests_equal(lhs_digest_string, rhs_digest_string):
+  """ Parse and compare the two digests, returns True if the digests are equal, False otherwise.
+  """
+  return parse_digest(lhs_digest_string) == parse_digest(rhs_digest_string)