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

port/stack_trace.cc

@@ -9,7 +9,7 @@
 #include <string.h>
 #include <unistd.h>
 
-namespace leveldb {
+namespace rocksdb {
 
 static const char* GetExecutableName()
 {
@@ -84,11 +84,11 @@ void InstallStackTraceHandler() {
 
 }
 
-}   // namespace leveldb
+}   // namespace rocksdb
 
 #else // no-op for non-linux system for now
 
-namespace leveldb {
+namespace rocksdb {
 
 void InstallStackTraceHandler() {}