revert fractional cascading in farward iterator
revert fractional cascading in farward iterator

Summary: As offline discussion with Siying, revert this since it has bug with seek.

Test Plan: make check -j64

Reviewers: yiwu, andrewkr, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D65559

Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,wenduo/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,wenduo/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,bbiao/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,wenduo/rocksdb