Sync titan directory when adding blob files (#136)

Summary:
When adding new blob file, titan directory need to be fsync, otherwise files can be lost after power outage.

Test Plan:
run `strace -yfe fsync ./titan_titan_db_test --gtest_filter=*.Snapshot` and see titan directory being fsync-ed in the output.
Signed-off-by: Yi Wu <yiwu@pingcap.com>

src/db_impl.cc

@@ -74,6 +74,10 @@ class TitanDBImpl::FileManager : public BlobFileManager {
                      Slice(file.first->largest_key()).ToString(true).c_str());
       edit.AddBlobFile(file.first);
     }
+    s = db_->directory_->Fsync();
+    if (!s.ok()) {
+      return s;
+    }
 
     {
       MutexLock l(&db_->mutex_);
@@ -232,6 +236,10 @@ Status TitanDBImpl::OpenImpl(const std::vector<TitanCFDescriptor>& descs,
   if (!s.ok()) {
     return s;
   }
+  s = env_->NewDirectory(dirname_, &directory_);
+  if (!s.ok()) {
+    return s;
+  }
   s = env_->LockFile(LockFileName(dirname_), &lock_);
   if (!s.ok()) {
     return s;

src/db_impl.h

@@ -256,6 +256,7 @@ class TitanDBImpl : public TitanDB {
   EnvOptions env_options_;
   DBImpl* db_impl_;
   TitanDBOptions db_options_;
+  std::unique_ptr<Directory> directory_;
 
   std::atomic<bool> initialized_{false};