[SPARK-19550][BUILD][CORE][WIP] Remove Java 7 support

- Move external/java8-tests tests into core, streaming, sql and remove
- Remove MaxPermGen and related options
- Fix some reflection / TODOs around Java 8+ methods
- Update doc references to 1.7/1.8 differences
- Remove Java 7/8 related build profiles
- Update some plugins for better Java 8 compatibility
- Fix a few Java-related warnings

For the future:

- Update Java 8 examples to fully use Java 8
- Update Java tests to use lambdas for simplicity
- Update Java internal implementations to use lambdas

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #16871 from srowen/SPARK-19493.

1 files changed, 0 insertions, 1 deletions

diff --git a/dev/mima b/dev/mima
index 11c4af2980..eca78ad109 100755
--- a/dev/mima
+++ b/dev/mima
@@ -31,7 +31,6 @@ OLD_DEPS_CLASSPATH="$(build/sbt -DcopyDependencies=false $SPARK_PROFILES "export
 rm -f .generated-mima*
 
 java \
-  -XX:MaxPermSize=1g \
   -Xmx2g \
   -cp "$TOOLS_CLASSPATH:$OLD_DEPS_CLASSPATH" \
   org.apache.spark.tools.GenerateMIMAIgnore