add disable WAL option

Summary: add disable WAL option

Test Plan: new testcase in db_test.cc

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D4011

include/leveldb/options.h

@@ -244,8 +244,13 @@ struct WriteOptions {
   // Default: false
   bool sync;
 
+  // If true, writes will not first go to the write ahead log,
+  // and the write may got lost after a crash.
+  bool disableWAL;
+
   WriteOptions()
-      : sync(false) {
+      : sync(false),
+        disableWAL(false) {
   }
 };