mapNextTx: use pointer as key, simplify value

Saves about 10% of application memory usage once the mempool warms up. Since the
mempool is DynamicUsage-regulated, this will translate to a larger mempool in
the same amount of space.

Map value type: eliminate the vin index; no users of the map need to know which
input of the transaction is spending the prevout.

Map key type: replace the COutPoint with a pointer to a COutPoint. A COutPoint
is 36 bytes, but each COutPoint is accessible from the same map entry's value.
A trivial DereferencingComparator functor allows indirect map keys, but the
resulting syntax is misleading: `map.find(&outpoint)`. Implement an indirectmap
that acts as a wrapper to a map that uses a DereferencingComparator, supporting
a syntax that accurately reflect the container's semantics: inserts and
iterators use pointers since they store pointers and need them to remain
constant and dereferenceable, but lookup functions take const references.

src/memusage.h

@@ -5,6 +5,8 @@
 #ifndef BITCOIN_MEMUSAGE_H
 #define BITCOIN_MEMUSAGE_H
 
+#include "indirectmap.h"
+
 #include <stdlib.h>
 
 #include <map>
@@ -106,6 +108,20 @@ static inline size_t IncrementalDynamicUsage(const std::map<X, Y, Z>& m)
     return MallocUsage(sizeof(stl_tree_node<std::pair<const X, Y> >));
 }
 
+// indirectmap has underlying map with pointer as key
+
+template<typename X, typename Y>
+static inline size_t DynamicUsage(const indirectmap<X, Y>& m)
+{
+    return MallocUsage(sizeof(stl_tree_node<std::pair<const X*, Y> >)) * m.size();
+}
+
+template<typename X, typename Y>
+static inline size_t IncrementalDynamicUsage(const indirectmap<X, Y>& m)
+{
+    return MallocUsage(sizeof(stl_tree_node<std::pair<const X*, Y> >));
+}
+
 // Boost data structures
 
 template<typename X>