handle kDelete type in cuckoo builder

Summary:
when I changed std::vector<std::string, std::string> to std::string to
store key/value pairs in builder, I missed the handling for kDeletion
type. As a result, value_size_ can be wrong if the first add key is for
deletion.
The is captured by ./cuckoo_table_db_test

Test Plan:
./cuckoo_table_db_test
./cuckoo_table_reader_test
./cuckoo_table_builder_test

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D24045