Fix -Wnon-virtual-dtor errors

Summary: This breaks mongo+rocks build https://mci.10gen.com/task_log_raw/rocksdb_ubuntu1404_rocksdb_c6e8e3d868660dc66b3bbd438cdc135df6356c5a_14_11_10_21_36_10_compile_ubuntu1404_rocksdb/0?type=T

Test Plan: m check + -Wnon-virtual-dtor

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28653