Summary:
Within this commit a new AbstractLogger was introduced
which allows to handle log messages at an application level.

Log messages are passed up to Java using a JNI callback.

This allows a Java-Developer to use common Java APIs for log
messages e.g. SLF4J, LOG4J, etc. Within this commit no new
dependencies were introduced, which keeps the RocksDB API clean
and doesn`t force a developer to use a predefined high-level Java API.

Another feature is to dynamically set a custom loggers verbosity at
runtime using its public method `setInfoLogLevel` and to retrieve
the currently active level using the `infoLogLevel` method.

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

Reviewers: adamretter, ankgup87, yhchiang

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D34755