clean up cluster_config format

Currently it looks something like this:

```
#
#Sat Aug 09 18:22:01 PDT 2014
peers=localhost\:5000,
version=0 -1
serverId=localhost\:5000
```

- We can remove the date. We can look at the file timestamp to find out when it got created.
- The version should be zxid, and it should probably be a part of the filename ("cluster_config.epoch.xid" for example).
- No need to escape colons.
- peers has a trailing comma.
- Maybe it's better to have one field called "servers". We can use the first server in the servers field as the local server id.