[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
OpenReadOnly support in RocksJava

Changes:
- new Method: RocksDB openReadOnly(String path)
- new Method: RocksDB openReadOnly(Options options, String path)

Missing:
- Column family support is blocked by #319