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/memtable.cc

@@ -14,7 +14,7 @@
 #include "util/coding.h"
 #include "util/murmurhash.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 MemTable::MemTable(const InternalKeyComparator& cmp,
                    std::shared_ptr<MemTableRepFactory> table_factory,
@@ -238,4 +238,4 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
   return false;
 }
 
-}  // namespace leveldb
+}  // namespace rocksdb