[RocksDB] Reduce memory footprint of the blockbased table hash index.

Summary:
Currently, the in-memory hash index of blockbased table uses a precise hash map to track the prefix to block range mapping. In some use cases, especially when prefix itself is big, the memory overhead becomes a problem. This diff introduces a fixed hash bucket array that does not store the prefix and allows prefix collision, which is similar to the plaintable hash index, in order to reduce the memory consumption.
Just a quick draft, still testing and refining.

Test Plan: unit test and shadow testing

Reviewers: dhruba, kailiu, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19047
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