Summary:
The recent Logger::Close() and DBImpl::Close() implementation rely on
calling the CloseImpl() virtual function from the destructor, which will
not work. Refactor the implementation to have a private close helper
function in derived classes that can be called by both CloseImpl() and
the destructor.
Closes https://github.com/facebook/rocksdb/pull/3528

Reviewed By: gfosco

Differential Revision: D7049303

Pulled By: anand1976

fbshipit-source-id: 76a64cbf403209216dfe4864ecf96b5d7f3db9f4