XEP-0198: Cosmetic change: Reuse event handler

On stanza queue overflow, pass a message to self() using the exclamation
mark operator instead of send_all_state_event/2.  This allows for
reusing the existing handler for 'kick' events.
Merge pull request #227 from weiss/xep-0198

XEP-0198: Cosmetic change: Reuse event handler