better sessionId generator 

change current sessionId generator:

``` java
UUID uuid = UUID.randomUUID();
String sid = Long.toString(Math.abs(uuid.getMostSignificantBits()), 36)
          + Long.toString(Math.abs(uuid.getLeastSignificantBits()), 36);
```

with

``` java
byte[] bytes = new byte[30];
rnd.nextBytes(bytes);
String sid = BaseEncoding.base64Url().encode(bytes);
```
