folder_branch_ops: check context during `syncAllUnlocked`
folder_branch_ops: check context during `syncAllUnlocked`

This fixes a race:

* CR input loop notices new work, creates a context, and starts a
  goroutine to do the CR.
* A write operation comes in from the user, grabs the mdWriter lock,
  finishes and calls cr.ForceCancel(), which cancels the CR context we
  just made.
* The CR goroutine now gets the mdWriter lock, and calls SyncAll,
  which includes the latest write.
* After the SyncAll finishes, it checks its context, sees that its
  canceled, and stops further processing.
* If no other writes happen, CR never starts up again.

Issue: KBFS-2505
Issue: keybase/client#8915

keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client