Fix WriteLevel0TableForRecovery file delete protection

Summary:
The call to

```
CaptureCurrentFileNumberInPendingOutputs()
```

should be before

```
versions_->NewFileNumber()
```
Right now we are not actually protecting the file from being deleted

Test Plan: make check

Reviewers: sdong, anthony, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D54645

db/db_impl.cc

@@ -1386,9 +1386,9 @@ Status DBImpl::WriteLevel0TableForRecovery(int job_id, ColumnFamilyData* cfd,
   mutex_.AssertHeld();
   const uint64_t start_micros = env_->NowMicros();
   FileMetaData meta;
-  meta.fd = FileDescriptor(versions_->NewFileNumber(), 0, 0);
   auto pending_outputs_inserted_elem =
       CaptureCurrentFileNumberInPendingOutputs();
+  meta.fd = FileDescriptor(versions_->NewFileNumber(), 0, 0);
   ReadOptions ro;
   ro.total_order_seek = true;
   Arena arena;

db/db_impl.h

@@ -494,10 +494,11 @@ class DBImpl : public DB {
 
   // Background process needs to call
   //     auto x = CaptureCurrentFileNumberInPendingOutputs()
+  //     auto file_num = versions_->NewFileNumber();
   //     <do something>
   //     ReleaseFileNumberFromPendingOutputs(x)
-  // This will protect any temporary files created while <do something> is
-  // executing from being deleted.
+  // This will protect any file with number `file_num` or greater from being
+  // deleted while <do something> is running.
   // -----------
   // This function will capture current file number and append it to
   // pending_outputs_. This will prevent any background process to delete any