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

table/block_builder.h

@@ -10,7 +10,7 @@
 #include <stdint.h>
 #include "rocksdb/slice.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 struct Options;
 
@@ -55,6 +55,6 @@ class BlockBuilder {
   void operator=(const BlockBuilder&);
 };
 
-}  // namespace leveldb
+}  // namespace rocksdb
 
 #endif  // STORAGE_LEVELDB_TABLE_BLOCK_BUILDER_H_