Flakey test: timestamp(com.microsoft.azure.cosmosdb.rx.DocumentCrudTest)

This test failed on a scheduled build. We should harden the logic or add retry:

```
[ERROR] timestamp(com.microsoft.azure.cosmosdb.rx.DocumentCrudTest) Time elapsed: 2.398 s <<< FAILURE!
java.lang.AssertionError:

Expecting:
<2019-06-04T10:55:36.000>
to be before or equals to:
<2019-06-04T10:55:35.927>
at com.microsoft.azure.cosmosdb.rx.DocumentCrudTest.timestamp(DocumentCrudTest.java:214)
```

Failing build examples:
- https://dev.azure.com/azure-cosmos-java/azure-cosmos-java/_build/results?buildId=944&view=logs&jobId=fd490c07-0b22-5182-fac9-6d67fe1e939b&taskId=420ba3bc-1a2f-58ee-a36c-596b8c354a2e&lineStart=20037&lineEnd=20044&colStart=1&colEnd=90