explore async checkpoints

[acunniffe]

Checkpoints don't need to actually do diffs, load blame and move around attributions. We could make them save all the current contents file blobs and relevant repo state, and wait until post-commit to start processing.

This could make commit overhead very minimal ~50ms (just dispatching a worker to process), but at the expense of having the graph ready.

We could also explore just handling moves async in the background so your new changes are labeled immediately and moved ones are handled after the fact