Deserialize custom Statistics object in db_bench
Deserialize custom Statistics object in db_bench

Summary:
Added -statistics_string to deserialize a Statistics object using the factory functions registered by applications.
Closes https://github.com/facebook/rocksdb/pull/1812

Differential Revision: D4469811

Pulled By: ajkr

fbshipit-source-id: 2d80862

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