Summary:
Simple Java Native Objects usually are represented using
the same functionality but within different classes.

With this commit a template class was introduced to remove
the redundant impelementation to a certain extent.

[RocksJava] Removed todo comment in portal.h

As jclass instances shall not be cached, both
todos are obsolete and can be removed.

[RocksJava] Add missing test to Makefile

[RocksJava] Added tests for uncovered methods

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

Reviewers: adamretter, yhchiang, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D32145