feat: Add "transfer_manager" module for concurrent uploads and downloads as a preview feature (#844)

* checkpoint before design doc impl

* checkpoint

* more tests

* code and tests for transfer manager complete

* proactively close temp files when finished reading

* respond to comments; destroy tmp files as they are consumed

* Add system tests, docstrings, address feedback

* Respond to review comments

* verify md5 hash of downloaded file in test

* lint

* default empty strings for root arguments

* fix bug with blob constructor

* add warning about files not being deleted if their downloads fail

Author: andrewsg

google/cloud/storage/constants.py

@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 """Constants used across google.cloud.storage modules."""
 
 # Storage classes

google/cloud/storage/fileio.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""Support for file-like I/O."""
+"""Module for file-like access of blobs, usually invoked via Blob.open()."""
 
 import io
 import warnings
@@ -101,10 +101,12 @@ class BlobReader(io.BufferedIOBase):
         - ``if_metageneration_match``
         - ``if_metageneration_not_match``
         - ``timeout``
+
+        Note that download_kwargs are also applied to blob.reload(), if a reload
+        is needed during seek().
     """
 
     def __init__(self, blob, chunk_size=None, retry=DEFAULT_RETRY, **download_kwargs):
-        """docstring note that download_kwargs also used for reload()"""
         for kwarg in download_kwargs:
             if kwarg not in VALID_DOWNLOAD_KWARGS:
                 raise ValueError(