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/table_cache.cc

@@ -11,7 +11,7 @@
 #include "util/coding.h"
 #include "util/stop_watch.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 static void DeleteEntry(const Slice& key, void* value) {
   Table* table = reinterpret_cast<Table*>(value);
@@ -160,4 +160,4 @@ void TableCache::Evict(uint64_t file_number) {
   cache_->Erase(Slice(buf, sizeof(buf)));
 }
 
-}  // namespace leveldb
+}  // namespace rocksdb