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

tools/shell/test/DBClientProxyTest.cpp

@@ -19,7 +19,7 @@
 
 
 #include "../DBClientProxy.h"
-using namespace leveldb;
+using namespace rocksdb;
 
 
 using namespace apache::thrift;