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/DBClientProxy.cpp

@@ -16,7 +16,7 @@ using namespace Tleveldb;
 using namespace apache::thrift::protocol;
 using namespace apache::thrift::transport;
 
-namespace leveldb {
+namespace rocksdb {
 
 DBClientProxy::DBClientProxy(const string & host, int port) :
   host_(host),