8460: Fix deadlock at shutdown by closing event stream before unmounting.

8460: Fix deadlock at shutdown by closing event stream before unmounting.

If llfuse shuts down while a thread is in a "with
llfuse.lock_released" block, the llfuse lock cannot be reacquired, so
the "with" block waits forever instead of exiting. The event listener
thread lands in this state easily because handling a "collection
updated" event makes network requests in a lock_released block.

This deadlock bug started occurring frequently in
tests.test_token_expiry.TokenExpiryTest when using arvados-ws.