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

db/builder.h

@@ -9,7 +9,7 @@
 #include "rocksdb/status.h"
 #include "rocksdb/types.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 struct Options;
 struct FileMetaData;
@@ -36,6 +36,6 @@ extern Status BuildTable(const std::string& dbname,
                          const SequenceNumber newest_snapshot,
                          const SequenceNumber earliest_seqno_in_memtable);
 
-}  // namespace leveldb
+}  // namespace rocksdb
 
 #endif  // STORAGE_LEVELDB_DB_BUILDER_H_