res_stasis: trigger cleanup after update

The cleanup code in stasis shuts down applications if they are in a deactivated
state, and no longer have explicit subscriptions. When registering an app the
cleanup code was running before calling 'update'. When it should be executed
after 'update' since a call to register may re-activate the app. We don't want
it to shutdown before the 'update' otherwise the app won't be re-activated,
or registered.

This patch makes it so the cleanup code is executed post 'update'.

ASTERISK-28679 #close

Change-Id: I8f2c0b17e33bb8128441567b97fd4c7bf74a327b