Filters getting disposed by System.gc before EOL

Previous to this commit Filters passed as parameters to the
BlockTableConfig are disposed before they should be disposed.

Further Smart pointer usage was corrected.

Java holds now the smart pointer to the FilterPolicy correctly
and cares about freeing underlying c++ structures.
Merge pull request #353 from fyrz/FixBloomfilterRocksJava

[RocksJava] Filters getting disposed by System.gc before EOL