Added naming of backup engine threads
Added naming of backup engine threads

Summary:
Changed the naming of backup engine threads from "ldb" to "backup_engine"
Closes https://github.com/facebook/rocksdb/pull/2053

Differential Revision: D4799325

Pulled By: ajkr

fbshipit-source-id: 046893f

SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB