LOGBACK-936 Update SyslogAppender to format days according to the spec



This pull request fixes the issue described in [LOGBACK-936](http://jira.qos.ch/browse/LOGBACK-936) where the dates in the Syslog messages were being formatted incorrectly according to the spec:
```
Feb 08 20:07:20 qk.local [Thread0] message
```
It now formats the dates like this:
```
Feb  8 20:31:25 qk.local [Thread0] message 
```

This pull request includes one other change: updating the caching scheme in the ```SyslogStartConverter``` to cache formatted strings for up to a second rather than up to a millisecond, since the millisecond value isn't part of the formatted string.