Fix valgrind error caused by FileMetaData as two level iterator's index block handle

Summary: It is a regression valgrind bug caused by using FileMetaData as index block handle. One of the fields of FileMetaData is not initialized after being contructed and copied, but I'm not able to find which one. Also, I realized that it's not a good idea to use FileMetaData as in TwoLevelIterator::InitDataBlock(), a copied FileMetaData can be compared with the one in version set byte by byte, but the refs can be changed. Also comparing such a large structure is slightly more expensive. Use a simpler structure instead

Test Plan:
Run the failing valgrind test (Harness.RandomizedLongDB)
make all check

Reviewers: igor, haobo, ljin

Reviewed By: igor

CC: yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D17409