Summary:
This renames the "Snapshot" op name to "Checkpoint" as we discussed earlier.
The early Snapshot name is still available, but we should move to the new name and
eventually deprecate the old name.
The Python SnapshotManager should be also changed, cc azzolini
Reviewed By: dzhulgakov
Differential Revision: D4272021
fbshipit-source-id: 4b8e029354416530dfbf0d538bfc91a0f61e0296
