Introduce src/reverse_iterator.hpp and include it...

...where it will be needed

Taken from https://gist.github.com/arvidsson/7231973 with small
modifications to fit the bitcoin core project

src/txmempool.cpp

@@ -11,6 +11,7 @@
 #include "validation.h"
 #include "policy/policy.h"
 #include "policy/fees.h"
+#include "reverse_iterator.h"
 #include "streams.h"
 #include "timedata.h"
 #include "util.h"