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.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/core.h b/src/core.h
index 27fda95552..8606831575 100644
--- a/src/core.h
+++ b/src/core.h
@@ -256,6 +256,9 @@ public:
         return hash;
     }
 
+    // True if only scriptSigs are different
+    bool IsEquivalentTo(const CTransaction& tx) const;
+
     // Return sum of txouts.
     int64_t GetValueOut() const;
     // GetValueIn() is a method on CCoinsViewCache, because