Initial effort to remove Guava dependency.

[thorbjornsen]

Replaced all references to Guava Optional with java.util.Optional. There were a few cases where direct translation/support is not available in Java 1.8 Optional class (or at all). java.util.Optional is not serializable, etc.

Note that there were two unit tests that were failing prior to the changes.

[xerial]

Thanks. This is a good first step. The next will be removing ImmutableList.of(...) usages

A key concern is serializing Optional.

I didn't know Optional in Java8 is non-serializable. It means Guava's Optional is much better for the current use cases in td-client-java. For example,td-spark needs to send table schema objects with Optional fields to remote worker nodes. So Optional fields must be serializable.

[xerial]

Spark is implementing their own Optional class. I think we should do a similar thing： https://github.com/apache/spark/blob/master/core/src/main/java/org/apache/spark/api/java/Optional.java

Instead of using java8's optional, we should implement our own like com.treasuredata.client.Optional

[thorbjornsen]

Per discussion here https://treasure-data.slack.com/archives/C03JNT0A6/p1564587329452600

Only two model classes have implemented the Serializable interface, TDColumn and TDColumnType. I improved the unit tests to ensure that the classes serialize and deserialize correctly. No need to overcomplicate things by implementing our own Optional class.

[xerial]

Superseded by #181