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/vectorrep.cc

@@ -11,7 +11,7 @@
 #include "util/mutexlock.h"
 #include "util/stl_wrappers.h"
 
-namespace leveldb {
+namespace rocksdb {
 namespace {
 
 using namespace stl_wrappers;
@@ -241,4 +241,4 @@ std::shared_ptr<MemTableRep> VectorRepFactory::CreateMemTableRep(
   MemTableRep::KeyComparator& compare, Arena* arena) {
   return std::make_shared<VectorRep>(compare, arena, count_);
 }
-} // namespace leveldb
+} // namespace rocksdb