Summary:
It's easy to cause coredump when closing ColumnFamilyHandle with unreleased iterators, especially iterators release is controlled by java GC when using JNI.

This patch fixed concurrent CF iteration and drop, we let iterators(actually SuperVersion) hold a ColumnFamilyData reference to prevent the CF from being released too early.

fixed https://github.com/facebook/rocksdb/issues/5982
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6147

Differential Revision: D18926378

fbshipit-source-id: 1dff6d068c603d012b81446812368bfee95a5e15