Change rollover event to be JSON

The log format for other pieces of data is JSON.
Change the rollover event to be consistent.  Also
do not record the previous name of the log file,
as that isn't used, and the current filename is
likely to change when it too gets rolled over
so there's no practical use of knowing the
no longer valid name for the transaction.

confluent_server/confluent/log.py

@@ -551,7 +551,7 @@ class Logger(object):
                 # which cause the rolling event.
                 to_bfile, to_tfile = files
                 self.logentries.appendleft(entry)
-                roll_data = "rename:%s>%s" % (self.handler.textpath, to_tfile)
+                roll_data = json.dumps({'previouslogfile': to_tfile})
                 self.logentries.appendleft([DataTypes.event, tstamp, roll_data,
                                             Events.logrollover, None])
             self._lock(fcntl.LOCK_UN)
@@ -563,7 +563,7 @@ class Logger(object):
 
         def parse_last_rolling_files(textfile, offset, datalen):
             textfile.seek(offset, 0)
-            textpath = textfile.read(datalen).split('>')[1]
+            textpath = json.loads(textfile.read(datalen))['previouslogfile']
             dir_name, base_name = os.path.split(textpath)
             temp = base_name.split('.')
             temp.insert(1,'cbl')