[RocksDB] Interface changes required for BackupableDB

Summary: This is part of https://reviews.facebook.net/D14295 -- smaller diff that is easier to review

Test Plan: make asan_check

Reviewers: dhruba, haobo, emayanke

Reviewed By: emayanke

CC: leveldb, kailiu, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14301

include/rocksdb/db.h

@@ -228,6 +228,9 @@ class DB {
   // Number of files in level-0 that would stop writes.
   virtual int Level0StopWriteTrigger() = 0;
 
+  // Get Env object from the DB
+  virtual Env* GetEnv() const = 0;
+
   // Flush all mem-table data.
   virtual Status Flush(const FlushOptions& options) = 0;