Important fixes to ring behaviour
Important fixes to ring behaviour

- Fixed reserve_span to correctly respond to ring resizes that occur while it's waiting.
- Fixed acquire_span to correctly handle span requests that can only be partially fulfilled.
- Added stream synchronize calls after memcopies to ensure they are synchronous wrt the host.

ledatelescope/bifrost,ledatelescope/bifrost,ledatelescope/bifrost,ledatelescope/bifrost