save.py: digest file writing thread bugs

[udim]

There are two bugs:

1. fast() is not waiting for the threads writing the digest and manifest.json files to complete. In my use case the digest file is left empty, which is due to bug no. 2.
2. After adding my fix (see below) I got this stacktrace (redacted in parts):

Traceback (most recent call last):
...
  File ".../containerregistry/client/v2_2/save.py", line 270, in fast
    future.result()
  File "<embedded stdlib>/concurrent/futures/_base.py", line 425, in result
    return self.__get_result()
  File "<embedded stdlib>/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "<embedded stdlib>/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File ".../containerregistry/client/v2_2/save.py", line 183, in write_file
    f.write(accessor(arg))
TypeError: a bytes-like object is required, not 'str'

Code in question:

containerregistry/client/v2_2/save_.py

Lines 221 to 225 in da03b39

	executor.submit(write_file, os.path.join(directory, 'digest'),
	lambda unused: image.digest(), 'unused')
	executor.submit(write_file, os.path.join(directory, 'manifest.json'),
	lambda unused: image.manifest().encode('utf8'),
	'unused')

My fix (note additional .encode('utf8'):

    digest_file = os.path.join(directory, 'digest')
    f = executor.submit(write_file, digest_file,
                        lambda unused: image.digest().encode('utf8'), 'unused')
    future_to_params[f] = digest_file
    manifest_file = os.path.join(directory, 'manifest.json')
    f = executor.submit(write_file, manifest_file,
                        lambda unused: image.manifest().encode('utf8'),
                        'unused')
    future_to_params[f] = manifest_file