[RocksJava] Quality improvements

Summary:
- Addressed some FindBugs issues.
- Remove obsolete dbFolder cleanup
- Comparator tests for CF
 - Added AbstractComparatorTest.
 - Fixed a bug in the JNI Part about Java comparators
- Minor test improvements

Test Plan:
make rocksdbjava
make jtest
mvn -f rocksjni.pom package

Reviewers: adamretter, yhchiang, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D29571
Fixed a Lint problem