Adopt a uniform toString() format

[pyokagan]

We currently have all kinds of toString() string representations in our code base. Making them uniform will help us easily identify object string representations and their containing data in places like logs, JUnit test failure messages etc.

Note, however, that some user-facing messages depend on the toString() representations. These should be migrated to use formatting methods that are specialized for user-facing messages instead.

Maybe use Guava's MoreObjects.toStringHelper()