Queue based async uploader
References #98
Test case is:
c = PodClient()
data = b'abc123' * 5000000
data2 = b'abd123' * 5000000
c.create(File(sha256=sha256(data).hexdigest())) # data file item
for i in range(100):
if i % 2 == 0:
c.upload_file_async(data, cb)
else:
c.upload_file_async(data2, cb)