Fix compaction bug in Cuckoo Table Builder. Use kvs_.size() instead of num_entries in FileSize() method.

Summary: Fix compaction bug in Cuckoo Table Builder. Use kvs_.size() instead of num_entries in FileSize() method. Also added tests.

Test Plan:
make check all
Also ran db_bench to generate multiple files.

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22743