master: allow filtering forwarded logs using logging package functions.

Given a message sent on "ssh.foo" to "mypkg.mymod", instead of logging
it to "mitogen.ctx.ssh.foo" in the master process, with the message
prefixed with the original logger name, instead log it to
"mypkg.mymod.[ssh.foo]", permitting normal logging package filtering
features to work as they usually do.

This also helps tidy up logging output a little bit.

mitogen/master.py

@@ -387,26 +387,26 @@ class LogForwarder(object):
         if msg.is_dead:
             return
 
-        logger = self._cache.get(msg.src_id)
-        if logger is None:
         context = self._router.context_by_id(msg.src_id)
         if context is None:
-                LOG.error('%s: dropping log from unknown context ID %d',
+            LOG.error('%s: dropping log from unknown context %d',
                       self, msg.src_id)
             return
 
-            name = '%s.%s' % (RLOG.name, context.name)
-            self._cache[msg.src_id] = logger = logging.getLogger(name)
-
         name, level_s, s = msg.data.decode('utf-8', 'replace').split('\x00', 2)
 
+        logger_name = '%s.[%s]' % (name, context.name)
+        logger = self._cache.get(logger_name)
+        if logger is None:
+            self._cache[logger_name] = logger = logging.getLogger(logger_name)
+
         # See logging.Handler.makeRecord()
         record = logging.LogRecord(
             name=logger.name,
             level=int(level_s),
             pathname='(unknown file)',
             lineno=0,
-            msg=('%s: %s' % (name, s)),
+            msg=s,
             args=(),
             exc_info=None,
         )