Implementing CuckooTableReader::NewIterator

Summary:
- Reads key-value pairs from file and builds an in-memory index of key-to-bucket id map in sorted order of key.
- Assumes bytewise comparator for sorting keys.
- Test changes

Test Plan:
cuckoo_table_reader_test --enable_perf
valgrind_check
asan_check

Reviewers: yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb, igor

Differential Revision: https://reviews.facebook.net/D20721