This fixes various race conditions and leaks related to the user's sync
and API event stream. It was possible for a sync/stream to begin after a
user was already closed; this change prevents that by managing the
goroutines related to sync/stream within cancellable groups.
We need to unlock the user keyring anyway to unlock the address keyring,
so we should just return it instead of re-unlocking the user keyring
when sending a message.
