[RocksJava] Make cleanup - Clean Target

- Remove JNI includes on clean
- Remove target folder generated by Maven
- Remove shared object
- Remove jar
[RocksJava] JavaDoc is executed too often

Previous to this commit too much targets got dependencies
on javadocs target.

Introduced one additional target "javalib" which resolves
that situation. JavaDoc will now be generated once while
executing a task with prefix "rocksdbjava".
[RocksJava] Strip library in publish

Currently maven publishing uses the library with debug symbols. What
leads to unnecessary big library sizes. Included strip to remove
unnecessary stuff. 40M -> 2.7M
Merge pull request #409 from fyrz/RocksJava-Make-cleanup

[RocksJava] Build improvements