add large size for servername

In tajo-core/src/main/java/org/apache/tajo/master/cluster/ServerName.java.
In getServerName, Creating StringBuilder With (hostName.length() + 4)

but like below, it is short, so it call internally newCapacity() if port size is larger than 3 
```java
     name.append(hostName); //+hostName.length()
     name.append(SERVERNAME_SEPARATOR); //+1
     name.append(port); //port range is 0 <= port range <= 65535
```

so this patch. prevent newCapacity() internally doing allocating proper size.

but. It just change value from 4 to 8(to cover port range 65535)
It passed testcases.