[RocksJava] Build improvements

This pull request includes 3 individual Java build improvements

1). Clean Target - clean up properly
- Remove JNI includes on clean
- Remove target folder generated by Maven
- Remove shared object
- Remove jar

2). JavaDoc target is executed far too often
Previous to this commit too much targets got dependencies
on javadocs target.

Introduced one additional target "compile" which resolves
that situation. JavaDoc will now be generated once while
executing a task with prefix "rocksdbjava".

3). Strip library to be published (Static and Release build)
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