Add a hash-index component for block

Summary:
this is the key component extracted from diff: https://reviews.facebook.net/D14271
I separate it to a dedicated patch to make the review easier.

Test Plan: added a unit test and passed it.

Reviewers: haobo, sdong, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16245

util/hash.cc

@@ -46,5 +46,4 @@ uint32_t Hash(const char* data, size_t n, uint32_t seed) {
   return h;
 }
 
-
 }  // namespace rocksdb