Change namespace from leveldb to rocksdb

Summary:
Change namespace from leveldb to rocksdb. This allows a single
application to link in open-source leveldb code as well as
rocksdb code into the same process.

Test Plan: compile rocksdb

Reviewers: emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13287

include/rocksdb/write_batch.h

@@ -24,7 +24,7 @@
 #include <string>
 #include "rocksdb/status.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 class Slice;
 
@@ -94,6 +94,8 @@ class WriteBatch {
   // Intentionally copyable
 };
 
-}  // namespace leveldb
+}  // namespace rocksdb
+
+#include "rocksdb/rocksdb_to_leveldb.h"
 
 #endif  // STORAGE_ROCKSDB_INCLUDE_WRITE_BATCH_H_