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/db_impl.cc

@@ -47,7 +47,7 @@
 #include "util/mutexlock.h"
 #include "util/stop_watch.h"
 
-namespace leveldb {
+namespace rocksdb {
 
 void dumpLeveldbBuildVersion(Logger * log);
 
@@ -3294,4 +3294,4 @@ void dumpLeveldbBuildVersion(Logger * log) {
       leveldb_build_compile_time, leveldb_build_compile_date);
 }
 
-}  // namespace leveldb
+}  // namespace rocksdb