Summary:
As you know, almost all compilers support "pragma once" keyword instead of using include guards. To be keep consistency between header files, all header files are edited.

Besides this, try to fix some warnings about loss of data.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4339

Differential Revision: D9654990

Pulled By: ajkr

fbshipit-source-id: c2cf3d2d03a599847684bed81378c401920ca848