More valgrind issues!

Summary: Fix some more CompactionFilterV2 valgrind issues. Maybe it would make sense for CompactionFilterV2 to delete its prefix_extractor?

Test Plan: ran CompactionFilterV2* tests with valgrind. issues before patch -> no issues after

Reviewers: haobo, sdong, ljin, dhruba

Reviewed By: dhruba

CC: leveldb, danguo

Differential Revision: https://reviews.facebook.net/D17337