srowen [SPARK-24421][BUILD][CORE] Accessing sun.misc.Cleaner in JDK11
…. Other related changes to get JDK 11 working, to test

## What changes were proposed in this pull request?

- Access `sun.misc.Cleaner` (Java 8) and `jdk.internal.ref.Cleaner` (JDK 9+) by reflection (note: the latter only works if illegal reflective access is allowed)
- Access `sun.misc.Unsafe.invokeCleaner` in Java 9+ instead of `sun.misc.Cleaner` (Java 8)

In order to test anything on JDK 11, I also fixed a few small things, which I include here:

- Fix minor JDK 11 compile issues
- Update scala plugin, Jetty for JDK 11, to facilitate tests too

This doesn't mean JDK 11 tests all pass now, but lots do. Note also that the JDK 9+ solution for the Cleaner has a big caveat.

## How was this patch tested?

Existing tests. Manually tested JDK 11 build and tests, and tests covering this change appear to pass. All Java 8 tests should still pass, but this change alone does not achieve full JDK 11 compatibility.

Closes #22993 from srowen/SPARK-24421.

Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
Latest commit 722369e Nov 14, 2018