XEP-0198: Log message when waiting for resumption

Log an informational message when a session goes into the pending state
(waiting for resumption) after the connection was lost.  Administrators
may well be interested in this state change when looking into issues.
XEP-0198: Accept stream elements in pending state

Due to timing issues, ejabberd_c2s might receive stream elements from
the client while the session is waiting for stream resumption.  Those
elements are now accepted.