We've had SLO for nearly ten years, and it is preferable for most
use-cases. It's time to default to SLO rather than DLO.
Add a new --use-dlo option to give users a way to return to old
behavior; ensure there is still a --use-slo option so we don't break
existing scripts that may use it.
UpgradeImpact:
==============
The default segmented-upload behavior has changed; Static Large Objects
are now used by default rather than Dynamic Large Objects. To revert to
the old behavior:
* CLI users may use the new `--use-dlo` option
* Service API users may explicitly set `use_slo` to False in their
options dicts
Change-Id: Iebcd64ced8eab6efebb9f02a0c4be6bf6338cb3b
6a8675e897