DeleteRange support for db_bench
DeleteRange support for db_bench

Summary:
Added a few options to configure when to add range tombstones during
any benchmark involving writes.
Closes https://github.com/facebook/rocksdb/pull/1522

Differential Revision: D4187388

Pulled By: ajkr

fbshipit-source-id: 2c8a473

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