Expose atomic flush option in C API (#6307)

Summary:
This PR adds a `rocksdb_options_set_atomic_flush` function to the C API.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6307

Differential Revision: D19451313

Pulled By: ltamasi

fbshipit-source-id: 750495642ef55b1ea7e13477f85c38cd6574849c

db/c.cc

@@ -2736,6 +2736,12 @@ void rocksdb_options_set_ratelimiter(rocksdb_options_t *opt, rocksdb_ratelimiter
   }
 }
 
+void rocksdb_options_set_atomic_flush(
+  rocksdb_options_t *opt,
+  unsigned char atomic_flush) {
+    opt->rep.atomic_flush = atomic_flush;
+}
+
 rocksdb_ratelimiter_t* rocksdb_ratelimiter_create(
     int64_t rate_bytes_per_sec,
     int64_t refill_period_us,

include/rocksdb/c.h

@@ -1037,6 +1037,8 @@ extern ROCKSDB_LIBRARY_API void rocksdb_options_set_fifo_compaction_options(
     rocksdb_options_t* opt, rocksdb_fifo_compaction_options_t* fifo);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_ratelimiter(
     rocksdb_options_t* opt, rocksdb_ratelimiter_t* limiter);
+extern ROCKSDB_LIBRARY_API void rocksdb_options_set_atomic_flush(
+    rocksdb_options_t* opt, unsigned char);
 
 /* RateLimiter */
 extern ROCKSDB_LIBRARY_API rocksdb_ratelimiter_t* rocksdb_ratelimiter_create(