Minor fix in rocksdb jni library, RocksDB now does not implement Closeable.

Summary:
* [java] RocksDB now does not implement Closeable.
* [java] RocksDB.close() is now synchronized.
* [c++] Fix a bug in rocksjni.cc that does not release a java reference before
        throwing an exception.

Test Plan:
make jni
make jtest

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17355

java/RocksDBSample.java

@@ -21,9 +21,10 @@ public class RocksDBSample {
     String db_path = args[0];
 
     System.out.println("RocksDBSample");
+    RocksDB db = null;
 
     try {
-      RocksDB db = RocksDB.open(db_path);
+      db = RocksDB.open(db_path);
       db.put("hello".getBytes(), "world".getBytes());
       byte[] value = db.get("hello".getBytes());
       System.out.format("Get('hello') = %s\n",
@@ -67,13 +68,11 @@ public class RocksDBSample {
       assert(len == RocksDB.NOT_FOUND);
       len = db.get(testKey, enoughArray);
       assert(len == testValue.length);
-      try {
-        db.close();
-      } catch (IOException e) {
-        System.err.println(e);
-      }
     } catch (RocksDBException e) {
       System.err.println(e);
     }
+    if (db != null) {
+      db.close();
+    }
   }
 }