untrusted stratum value for TrueTime: 0 ?

No idea what triggers it, but occasionally, TrueTimeRx fails to initialize with a `untrusted stratum value for TrueTime: 0`. I am using the `pool.ntp.org` pool.

After a few hours, the issue just goes away, but it's usually back soon enough. Looking at the logs, my guess about what happens is that TrueTimeRx only does requests to a single server (`193.70.94.182` in this case) again and again, even though it is misbehaving. Perhaps it would be better to retry with a different server instead of wasting all attempts on a single one?

Code I use:
```
mTrueTime =
TrueTimeRx
.build()
.withLoggingEnabled(true)
.withConnectionTimeout(15 * 1000)
.withServerResponseDelayMax(1000)
.withRootDispersionMax(1500)
.withRootDelayMax(1500)
.withSharedPreferencesCache(context);
```

```
mTrueTime
.initializeRx("pool.ntp.org")
.subscribeOn(Schedulers.io())
.subscribe(...)
```

Full log: [log.txt](https://github.com/instacart/truetime-android/files/4003429/log.txt)