Relay double-spends, subject to anti-DOS

Allows network wallets and other clients to see transactions that respend
a prevout already spent in an unconfirmed transaction in this node's mempool.

Knowledge of an attempted double-spend is of interest to recipients of the
first spend.  In some cases, it will allow these recipients to withhold
goods or services upon being alerted of a double-spend that deprives them
of payment.

As before, respends are not added to the mempool.

Anti-Denial-of-Service-Attack provisions:
 - Use a bloom filter to relay only one respend per mempool prevout
 - Rate-limit respend relays to a default of 100 thousand bytes/minute
 - Define tx2.IsEquivalentTo(tx1): equality when scriptSigs are not considered
 - Do not relay these equivalent transactions

Remove an unused variable declaration in txmempool.cpp.

src/txmempool.cpp

@@ -415,7 +415,6 @@ void CTxMemPool::remove(const CTransaction &tx, std::list<CTransaction>& removed
 void CTxMemPool::removeConflicts(const CTransaction &tx, std::list<CTransaction>& removed)
 {
     // Remove transactions which depend on inputs of tx, recursively
-    list<CTransaction> result;
     LOCK(cs);
     BOOST_FOREACH(const CTxIn &txin, tx.vin) {
         std::map<COutPoint, CInPoint>::iterator it = mapNextTx.find(txin.prevout);