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.