XEP-0198: Cosmetic change: Simplify state change

When the FSM goes into the 'wait_for_resume' state, let fsm_next_state/2
take care of updating #state.mgmt_state and of writing the log line.
This doesn't change the behavior, but simplifies the code.
Merge pull request #226 from weiss/simplify-state-change

XEP-0198: Cosmetic change: Simplify state change