db_stress support for range deletions
db_stress support for range deletions

Summary:
made db_stress capable of adding range deletions to its db and verifying their correctness. i'll make db_crashtest.py use this option later once the collapsing optimization (https://github.com/facebook/rocksdb/pull/1614) is committed because currently it slows down the test too much.
Closes https://github.com/facebook/rocksdb/pull/1625

Differential Revision: D4293939

Pulled By: ajkr

fbshipit-source-id: d3beb3a

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