MXS-421 Add maxscale::event concept

MaxScale now defines events for which the syslog
facility and level can explicitly be defined by the
administrator. Currently there is only one such
event, namelt AUTHENTICATION_FAILURE.

In a subsequent commit, config.cc will be modified so
that event-related configuration parameters are passed
to event::configure() and in another subsequent commit
the authenticators will be modifed to use this mechanism.

In practice a line like:

   MXS_WARNING("%s: login attempt for user '%s'@[%s]:%s, "
               "authentication failed.",
               dcb->service->name, client_data->user,
               dcb->remote, dcb->path);

will be changed to

    MXS_LOG_EVENT(event::AUTHENTICATION_FAILURE,
                  "%s: login attempt for user '%s'@[%s]:%s, "
                  "authentication failed.",
                  dcb->service->name, client_data->user,
                  dcb->remote, dcb->path);

server/core/CMakeLists.txt

@@ -10,6 +10,7 @@ add_library(maxscale-common SHARED
   config_runtime.cc
   dcb.cc
   encryption.cc
+  event.cc
   externcmd.cc
   filter.cc
   hashtable.cc