Add C API to set max_file_opening_threads option

Summary:
Add `rocksdb_options_set_max_file_opening_threads()` API
Closes https://github.com/facebook/rocksdb/pull/2184

Differential Revision: D4923090

Pulled By: lightmark

fbshipit-source-id: c4ddce17733d999d426d02f7202b33a46ed6faed

db/c.cc

@@ -1989,6 +1989,10 @@ void rocksdb_options_set_max_open_files(rocksdb_options_t* opt, int n) {
   opt->rep.max_open_files = n;
 }
 
+void rocksdb_options_set_max_file_opening_threads(rocksdb_options_t* opt, int n) {
+  opt->rep.max_file_opening_threads = n;
+}
+
 void rocksdb_options_set_max_total_wal_size(rocksdb_options_t* opt, uint64_t n) {
   opt->rep.max_total_wal_size = n;
 }

include/rocksdb/c.h

@@ -707,6 +707,8 @@ extern ROCKSDB_LIBRARY_API void rocksdb_options_set_db_write_buffer_size(
     rocksdb_options_t*, size_t);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_max_open_files(
     rocksdb_options_t*, int);
+extern ROCKSDB_LIBRARY_API void rocksdb_options_set_max_file_opening_threads(
+    rocksdb_options_t*, int);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_max_total_wal_size(
     rocksdb_options_t* opt, uint64_t n);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_compression_options(