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

util/ldb_cmd.cc

@@ -18,7 +18,7 @@
 #include <string>
 #include <stdexcept>
 
-namespace leveldb {
+namespace rocksdb {
 
 using namespace std;