[FLINK-3195] [examples] Consolidate Java/Scala and batch/streaming examples

[StephanEwen]

This pull request consolidates the examples:

• One root project flink-examples
• Subprojects flink-examples-batch and flink-examples-streaming
• Batch examples are put into the distribution under examples/batch
• Streaming examples are now also put into the distribution under examples/streaming
• Java / Scala examples are always in one project. Since Java code never references Scala code, this should be okay, even for Eclipse
• Removed redundant jars, such as WordCoundPOJO, and EnumTrianglesOpt. Since the JARs do not add any visible difference, they are not necessary and only confusing.
• Rename PageRankBasic to PageRank

Open question: Moving the final example jars to examples/batch breaks a few web tutorials. Since these tutorials usually mention a Flink version (and this is simple to figure out for users), I think it is okay.

An alternative is to have all examples in the /examples folder and changing the names to make them unique, for example WordCount.jar and StreamingWordCount.jar.

[fhueske]

Edge, EdgeWithDegrees, and Triad should be added back to the imports to make the code more readable.

[fhueske]

I like the restructuring. I am also fine with remove redundant examples. However, I think we should either not rename the triangle enumeration jar to EnumerateGraphTriangles.jar or also rename the source file. Otherwise it is not clear which code is contained in the jar. I would also be OK with removing the EnumTriangleOpt example completely.

I think it is OK to move the batch example to a subfolder. They should be rather easy to find.

[rmetzger]

I'd like to finish the project restructure and I'll use this PR as a base. I'll also address Fabian's comments.