Fix BYTES_WRITTEN accounting
Fix BYTES_WRITTEN accounting

Summary:
BYTES_WRITTEN accounting doesn't work with disabled WAL. For example, this is what we
get in the LOG:

```
Cumulative writes: 9794K writes, 228M keys, 9794K commit groups, 1.0
writes per commit group, ingest: 0.00 GB, 0.00 MB/s
```

WAL bytes are tracked in a different statistic:
https://github.com/facebook/rocksdb/blob/master/db/internal_stats.h#L105.
BYTES_WRITTEN should count all the writes.
Closes https://github.com/facebook/rocksdb/pull/2133

Differential Revision: D4880615

Pulled By: yiwu-arbug

fbshipit-source-id: 8fd0b223099f3f5ad7df79d4e737d313687fec69

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