configBackwardCompatibilityMode breaks compatibility

The current implementation divide information of servers of legacy config into two separate dynamic config files. There is a problem.

When we set "clientPort" variable in config file, it gets automatically erased and later on there is no information about "clientPort" in either the old or new (.dynamic) config file.

It becomes a serious problem when users of 3.4 jump to 3.5 directly without changing their config: when a server crashes and restarts, there is no client port serving.

For example,
a legacy config might look like:

```zoo.cfg
dataDir=/root/zookeeper/groupconfig/conf1/data
syncLimit=5
initLimit=10
tickTime=2000
clientPort=2181
server.1=127.0.0.1:2222:2223
server.2=127.0.0.1:3333:3334
server.3=127.0.0.1:4444:4445
```

After dynamic reconfig, it might look like

```zoo.cfg
dataDir=/root/zookeeper/groupconfig/conf1/data
syncLimit=5
tickTime=2000
initLimit=10
dynamicConfigFile=./zoo.cfg.dynamic
```

and
```zoo.cfg.dynamic
server.1=127.0.0.1:2222:2223:participant
server.2=127.0.0.1:3333:3334:participant
server.3=127.0.0.1:4444:4445:participant
version=e00000000
```

This could be successfully started at first time. But when server restarts from crash, it never serve client port again.