Use a different approach to make sure BlockBasedTableReader can use hash index on older files

Summary:
A recent commit https://github.com/facebook/rocksdb/commit/e37dd216f9384bfdabc6760fa296e8ee28c79d30 makes sure hash index can be used when reading existing files. This patch uses another way to achieve the approach:
(1) Currently, always writing kBinarySearch to files, despite of BlockBasedTableOptions.IndexType setting.
(2) When reading a file, read out the field, and make sure it is kBinarySearch, while always use index type by users.

The reason for doing it is, to reserve kHashSearch property on disk to future. If now we write out binary index for both of kHashSearch and kBinarySearch. We have to use a new flag in the future for hash index on disk, otherwise compatibility would break. Also, we want the real index type and type shown in properties block to be consistent.

Test Plan: make all check

Reviewers: haobo, kailiu

Reviewed By: kailiu

CC: igor, ljin, yhchiang, xjin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18009