[RocksDB Performance Branch] A more customized index in PlainTableReader

Summary:
PlainTableReader to use a more customized hash table. This patch assumes the SST file is smaller than 2GB:
(1) Every bucket uses 32-bit integer
(2) no key is stored in bucket
(3) use the first bit of the bucket value to distinguish it points to the file offset or a second level index.
This index schema fits the use case that most of prefixes have very small number of keys

Test Plan: plain_table_db_test

Reviewers: haobo, kailiu, dhruba

Reviewed By: haobo

CC: nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D14343

include/rocksdb/plain_table_factory.h

@@ -23,7 +23,9 @@ class TableBuilder;
 
 // IndexedTable requires fixed length key, configured as a constructor
 // parameter of the factory class. Output file format:
-// +--------------------------------------------+  <= key1 offset
+// +-------------+
+// | version     |
+// +-------------+------------------------------+  <= key1 offset
 // | key1            | value_size (4 bytes) |   |
 // +----------------------------------------+   |
 // | value1                                     |