Make it possible to use LevelDB native binary installed by OS package manager, instead of the one bundled in leveldbjni-all

Some downstream distributions (I know of at least one..) object to having native .so binaries bundled in Java JAR, such as the /META-INF/native//*/libleveldbjni.so in leveldbjni-all-1.8-odl.jar, and would prefer to be able to use a LevelDB native binary installed by OS package manager (e.g. RPM).

The as-is way is convenient though as default, so this should be more like an option: e.g. a -D or a sep. Karaf feature or even just a fall-back built into leveldbjni - like if no suitable /META-INF/native///libleveldbjni.so found, then attempt to find and use a LevelDB native binary installed by OS package manager.