Let ejabberd_c2s always use fsm_next_state/2

Make sure any logic implemented in ejabberd_c2s:fsm_next_state/2 is
always applied.
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 #225 from weiss/use-fsm-next-state

Let ejabberd_c2s always use fsm_next_state/2
Merge pull request #226 from weiss/simplify-state-change

XEP-0198: Cosmetic change: Simplify state change
XEP-0198: Terminate session on queue overflow

On queue overflow, terminate the c2s session instead of just dropping
items from the queue.  This makes sure all stanzas are either delivered
or bounced.