refactor: 'checkpoint' to 'memory snapshot' renaming

[thundergolfer]

Describe your changes

One more step on the road to renaming checkpoint to memory snapshot.

Backward/forward compatibility checks

Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
  • Should be pure refactor. None of these name/symbol changes cross the boundary between the client and the worker/server/host.
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself
  • Should be pure refactor. None of these name/symbol changes cross the boundary between the client and the worker/server/host.

Changelog

[mwaskom]

👍