MINOR: spoe: Add counters to log info about SPOE agents

MINOR: spoe: Add counters to log info about SPOE agents

In addition to metrics about time spent in the SPOE, following counters have
been added:

  * applets : number of SPOE applets.
  * idles : number of idle applets.
  * nb_sending : number of streams waiting to send data.
  * nb_waiting : number of streams waiting for a ack.
  * nb_processed : number of events/groups processed by the SPOE (from the
                   stream point of view).
  * nb_errors : number of errors during the processing (from the stream point of
                view).

Log messages has been updated to report these counters. Following pattern has
been added at the end of the log message:

    ... <idles>/<applets> <nb_sending>/<nb_waiting> <nb_error>/<nb_processed>