Fix Fetch -> FetchStreaming conversion

As blob.fetcherToSeekerWrapper.Fetch erroneously asserts that FetchStreaming
returns a ReadSeekCloser everytime, it had to be changed.

Move MaxBlobSize from blobserver to constants (new package).

Change-Id: I4b4f22c302cbec84d77d21454e0c9e8aebdf73e5