Summary:
Add SyncWAL to db_stress. Specify with `-sync_wal_one_in=N` so that it will be
called once every N operations on average.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6149
Test Plan:
```
$make db_stress
$./db_stress -sync_wal_one_in=100 -ops_per_thread=100000
```
Differential Revision: D18922529
Pulled By: riversand963
fbshipit-source-id: 4c0b8cb8fa21852722cffd957deddf688f12ea56