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/txmempool.h

@@ -11,6 +11,7 @@
 
 #include "amount.h"
 #include "coins.h"
+#include "indirectmap.h"
 #include "primitives/transaction.h"
 #include "sync.h"
 
@@ -305,20 +306,6 @@ struct ancestor_score {};
 
 class CBlockPolicyEstimator;
 
-/** An inpoint - a combination of a transaction and an index n into its vin */
-class CInPoint
-{
-public:
-    const CTransaction* ptx;
-    uint32_t n;
-
-    CInPoint() { SetNull(); }
-    CInPoint(const CTransaction* ptxIn, uint32_t nIn) { ptx = ptxIn; n = nIn; }
-    void SetNull() { ptx = NULL; n = (uint32_t) -1; }
-    bool IsNull() const { return (ptx == NULL && n == (uint32_t) -1); }
-    size_t DynamicMemoryUsage() const { return 0; }
-};
-
 /**
  * CTxMemPool stores valid-according-to-the-current-best-chain
  * transactions that may be included in the next block.
@@ -477,7 +464,7 @@ private:
     void UpdateChild(txiter entry, txiter child, bool add);
 
 public:
-    std::map<COutPoint, CInPoint> mapNextTx;
+    indirectmap<COutPoint, const CTransaction*> mapNextTx;
     std::map<uint256, std::pair<double, CAmount> > mapDeltas;
 
     /** Create a new CTxMemPool.