Better interface to create BackupEngine

Summary: I think it looks nicer. In RocksDB we have both styles, but I think that static method is the more common version.

Test Plan: backupable_db_test

Reviewers: ljin, benj, swk

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D15519

include/utilities/backupable_db.h

@@ -85,6 +85,9 @@ class BackupEngine {
  public:
   virtual ~BackupEngine() {}
 
+  static BackupEngine* NewBackupEngine(Env* db_env,
+                                       const BackupableDBOptions& options);
+
   virtual Status CreateNewBackup(DB* db, bool flush_before_backup = false) = 0;
   virtual Status PurgeOldBackups(uint32_t num_backups_to_keep) = 0;
   virtual Status DeleteBackup(BackupID backup_id) = 0;
@@ -100,9 +103,6 @@ class BackupEngine {
   virtual void DeleteBackupsNewerThan(uint64_t sequence_number) = 0;
 };
 
-extern BackupEngine* CreateNewBackupEngine(Env* db_env,
-                                           const BackupableDBOptions& options);
-
 // Stack your DB with BackupableDB to be able to backup the DB
 class BackupableDB : public StackableDB {
  public:

utilities/backupable/backupable_db.cc

@@ -188,8 +188,8 @@ class BackupEngineImpl : public BackupEngine {
   static const size_t copy_file_buffer_size_ = 5 * 1024 * 1024LL; // 5MB
 };
 
-BackupEngine* CreateNewBackupEngine(Env* db_env,
+BackupEngine* BackupEngine::NewBackupEngine(
-                                    const BackupableDBOptions& options) {
+    Env* db_env, const BackupableDBOptions& options) {
   return new BackupEngineImpl(db_env, options);
 }