Summary: It used to be no good (known to me) non-intrusive way to wrap WritableFile - you can't call protected virtual methods of the wrapped pointer to WritableFile. This diff adds a convenience class WritableFileWrapper that makes wrapping WritableFile both possible and easy.

Test Plan: `make clean; make -j release`, `make clean; OPT=-DROCKSDB_LITE make release`, `make clean; USE_CLANG=1 make -j all`.

Reviewers: sdong, yhchiang, rven

Reviewed By: rven

Subscribers: dhruba, tnovak, march

Differential Revision: https://reviews.facebook.net/D39147