[Java] Fixed link error on library loading on Mac.

Summary:
Fixed link error on library loading on Mac.

Test Plan:
make rocksdbjava
make jtest
[Java] Fix compile error on DbBenchmark.java

Summary:
Fix compile error on DbBenchmark.java

Test Plan:
make rocksdbjava
make jdb_bench
Fixed Findbugs issues

- BackupableDB missing call to super.finalize(major)
- WriteBatchTest inefficient String usage(minor)
- RocksDB local dead variable store(medium)