PlainTable to encode to avoid to rewrite prefix when it is the same as the previous key

Summary:
Add a encoding feature of PlainTable to encode PlainTable's keys to save some bytes for the same prefixes.
The data format is documented in table/plain_table_factory.h

Test Plan: Add unit test coverage in plain_table_db_test

Reviewers: yhchiang, igor, dhruba, ljin, haobo

Reviewed By: haobo

Subscribers: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D18735