Forward port form 2.7 branch.

medusa's class monitor_server derives from asyncore.dispatcher, but
never called the latter's constructor.  As a result, the ._map attribute
set by 2.4's asyncore.dispatcher.__init__() never gets set, and calling
asyncore methods later dies with an AttributeError on ._map.  The fix
is a 1-liner, adding a call to the base-class constructor.