folder_branch_ops: fix timer logic
folder_branch_ops: fix timer logic

The rekeyWithPrompt timer should only ever be set once, and it should
be stopped once the rekey is no longer needed.

Issue: KBFS-937
Issue: #1007

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