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/main.cpp

@@ -1049,9 +1049,10 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
     LOCK(pool.cs); // protect pool.mapNextTx
     BOOST_FOREACH(const CTxIn &txin, tx.vin)
     {
-        if (pool.mapNextTx.count(txin.prevout))
+        auto itConflicting = pool.mapNextTx.find(txin.prevout);
+        if (itConflicting != pool.mapNextTx.end())
         {
-            const CTransaction *ptxConflicting = pool.mapNextTx[txin.prevout].ptx;
+            const CTransaction *ptxConflicting = itConflicting->second;
             if (!setConflicts.count(ptxConflicting->GetHash()))
             {
                 // Allow opt-out of transaction replacement by setting