Fix readonly db

Summary:
DBImplReadOnly::CompactRange wasn't override DBImpl::CompactRange;
this can cause problem when using StackableDB inheritors like
DbWithTtl.
P. S. Thanks C++11 for override :)

Test Plan: make all check

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D20829