AVRO-1047: Java: Fix remaining javac warnings.

[mkw]

AVRO-1913 fixed most warnings, but missed a few unchecked usages and left in some redundant casts. This patch eliminates remaining warnings even with the -Xlint:all flag added to javac. With no more warnings to surpress, the @SuppressWarnings("all") annotation isremoved from the generated code, as well.

[nielsbasjes]

First impression of this change is good.
Things that need attention:

1. Can you check if it is easily possible to force the regular build to compile the generated code with this -Xlint:all ? That way we can spot new problems in this area much faster.
2. When I run the full build (i.e. mvn clean package) I get test errors that the newly generated classes do not match the versions in the code base. (Position.java and Player.java) This is a normal effect of changing the code generation.
3. This improvement is not mentioned yet in the CHANGES.txt

[mkw]

I modified org.apache.avro.compiler.specific.TestSpecificCompiler#assertCompilesWithJavaCompiler to catch these in tests:

1. Passed -Xlint:all to the compile task.
2. Added a compiler diagnostic listener to collect warnings and an assert that fails the test when present.
3. Added a test schema that triggers the warnings that were fixed by the original commit (union&fixed types).

Also fixed the failing tests, added a summary in CHANGES.txt, rebased off of branch-1.8, and squashed the history into a single commit. Let me know if I should do anything else!

[nielsbasjes]

+1
Thanks for this cleanup!

[nielsbasjes]

Committed to both branch-1.8 and master.
Thanks.

(Can you please close this pull request? Thanks.)