resolution for java build problem introduced by 5ec53f3edf62bec1b690ce12fb21a6c52203f3c8
Fixed compile warning on Mac caused by unused variables.

Summary:
Fixed compile warning caused by unused variables.

./db/compaction_picker.h:118:7: error: private field 'max_grandparent_overlap_factor_' is not used [-Werror,-Wunused-private-field]
  int max_grandparent_overlap_factor_;
      ^
./db/compaction_picker.h:119:7: error: private field 'expanded_compaction_factor_' is not used [-Werror,-Wunused-private-field]
  int expanded_compaction_factor_;
      ^
2 errors generated.

Test Plan:
make db_test
Fixed signed-unsigned comparison warning in db_test.cc

Summary:
Fixed signed-unsigned comparison warning in db_test.cc

db/db_test.cc:8606:3: note: in instantiation of function template specialization 'rocksdb::test::Tester::IsEq<int, unsigned long>' requested here
  ASSERT_EQ(2, metadata.size());
    ^

Test Plan:
make db_test
Merge pull request #329 from fyrz/master

resolution for java build problem introduced by 5ec53f3edf62bec1b690ce12...
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