Explicitly declare atomic flush incompatible with pipelined write (#5860)

Summary:
Atomic flush is incompatible with pipelined write. At least now.
If pipelined write is enabled, a thread performing write can exit the write
thread and start inserting into memtables. Consequently a thread performing
flush will enter write thread and race with memtable insertion by the former.
This will cause undefined result in terms of data persistence.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5860

Test Plan:
```
$make all && make check
```

Differential Revision: D17638944

Pulled By: riversand963

fbshipit-source-id: abc578dc49a5dbe41bc5adcecf448f8e042a6d49

db/db_impl/db_impl_open.cc

@@ -228,6 +228,11 @@ Status DBImpl::ValidateOptions(const DBOptions& db_options) {
         "unordered_write is incompatible with enable_pipelined_write");
   }
 
+  if (db_options.atomic_flush && db_options.enable_pipelined_write) {
+    return Status::InvalidArgument(
+        "atomic_flush is incompatible with enable_pipelined_write");
+  }
+
   return Status::OK();
 }